diff --git a/applications/test/parallel-comm1/Test-parallel-comm1.C b/applications/test/parallel-comm1/Test-parallel-comm1.C
index ed8ce38516..d0bc966a5d 100644
--- a/applications/test/parallel-comm1/Test-parallel-comm1.C
+++ b/applications/test/parallel-comm1/Test-parallel-comm1.C
@@ -50,37 +50,37 @@ scalar sumReduce
 )
 {
     scalar sum = 0;
-    if (Pstream::parRun())
+    if (UPstream::parRun())
     {
         if (UPstream::master(comm))
         {
-            // Add master value and all slaves
+            // Add master value and all sub-procs
             sum = localValue;
 
-            for (const int slave : UPstream::subProcs(comm))
+            for (const int proci : UPstream::subProcs(comm))
             {
-                scalar slaveValue;
+                scalar procValue;
                 UIPstream::read
                 (
-                    Pstream::commsTypes::blocking,
-                    slave,
-                    reinterpret_cast<char*>(&slaveValue),
+                    UPstream::commsTypes::blocking,
+                    proci,
+                    reinterpret_cast<char*>(&procValue),
                     sizeof(scalar),
                     UPstream::msgType(),    // tag
                     comm                    // communicator
                 );
 
-                sum += slaveValue;
+                sum += procValue;
             }
 
-            // Send back to slaves
+            // Send back
 
-            for (const int slave : UPstream::subProcs(comm))
+            for (const int proci : UPstream::subProcs(comm))
             {
                 UOPstream::write
                 (
                     UPstream::commsTypes::blocking,
-                    slave,
+                    proci,
                     reinterpret_cast<const char*>(&sum),
                     sizeof(scalar),
                     UPstream::msgType(),    // tag
diff --git a/applications/test/parallel/Test-parallel.C b/applications/test/parallel/Test-parallel.C
index 580ae453a8..7479a49922 100644
--- a/applications/test/parallel/Test-parallel.C
+++ b/applications/test/parallel/Test-parallel.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2020 OpenCFD Ltd.
+    Copyright (C) 2016-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -52,27 +52,27 @@ void testMapDistribute()
 
     // Generate random data.
     List<Tuple2<label, List<scalar>>> complexData(100);
-    forAll(complexData, i)
+    for (auto& data : complexData)
     {
-        complexData[i].first() = rndGen.position(0, Pstream::nProcs()-1);
-        complexData[i].second().setSize(3);
-        complexData[i].second()[0] = 1;
-        complexData[i].second()[1] = 2;
-        complexData[i].second()[2] = 3;
+        data.first() = rndGen.position(0, UPstream::nProcs()-1);
+        data.second().resize(3);
+        data.second()[0] = 1;
+        data.second()[1] = 2;
+        data.second()[2] = 3;
     }
 
     // Send all ones to processor indicated by .first()
 
     // Count how many to send
-    labelList nSend(Pstream::nProcs(), Zero);
-    forAll(complexData, i)
+    labelList nSend(UPstream::nProcs(), Foam::zero{});
+    for (const auto& data : complexData)
     {
-        const label proci = complexData[i].first();
+        const label proci = data.first();
         nSend[proci]++;
     }
 
     // Collect items to be sent
-    labelListList sendMap(Pstream::nProcs());
+    labelListList sendMap(UPstream::nProcs());
     forAll(sendMap, proci)
     {
         sendMap[proci].resize_nocopy(nSend[proci]);
@@ -116,40 +116,31 @@ void testTransfer(const T& input)
 {
     T data = input;
 
-    if (Pstream::master())
+    if (UPstream::master())
     {
         Perr<<"test transfer (" << (typeid(T).name()) << "): ";
         perrInfo(data) << nl << endl;
-    }
 
-    if (Pstream::master())
-    {
-        for (const int slave : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            Perr<< "master receiving from slave " << slave << endl;
-            IPstream fromSlave(Pstream::commsTypes::blocking, slave);
-            fromSlave >> data;
+            Perr<< "master receiving from proc:" << proci << endl;
+            IPstream::recv(data, proci);
             perrInfo(data) << endl;
         }
 
-        for (const int slave : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            Perr<< "master sending to slave " << slave << endl;
-            OPstream toSlave(Pstream::commsTypes::blocking, slave);
-            toSlave << data;
+            Perr<< "master sending to proc:" << proci << endl;
+            OPstream::bsend(data, proci);
         }
     }
     else
     {
-        {
-            Perr<< "slave sending to master " << Pstream::masterNo() << endl;
-            OPstream toMaster(Pstream::commsTypes::blocking, Pstream::masterNo());
-            toMaster << data;
-        }
+        Perr<< "proc sending to master" << endl;
+        OPstream::bsend(data, UPstream::masterNo());
 
-        Perr<< "slave receiving from master " << Pstream::masterNo() << endl;
-        IPstream fromMaster(Pstream::commsTypes::blocking, Pstream::masterNo());
-        fromMaster >> data;
+        Perr<< "proc receiving from master" << endl;
+        IPstream::recv(data, UPstream::masterNo());
         perrInfo(data) << endl;
     }
 }
@@ -160,49 +151,31 @@ void testTokenized(const T& data)
 {
     token tok;
 
-    if (Pstream::master())
+    if (UPstream::master())
     {
-        Perr<<"test tokenized \"" << data << "\"" << nl << endl;
-    }
+        Perr<< "test tokenized \"" << data << "\"" << nl << endl;
 
-    if (Pstream::master())
-    {
-        for (const int slave : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            Perr<< "master receiving from slave " << slave << endl;
-            IPstream fromSlave(Pstream::commsTypes::blocking, slave);
-            fromSlave >> tok;
+            Perr<< "master receiving from proc:" << proci << endl;
+            IPstream::recv(tok, proci);
             Perr<< tok.info() << endl;
         }
 
-        for (const int slave : Pstream::subProcs())
+        for (const int proci : UPstream::subProcs())
         {
-            Perr<< "master sending to slave " << slave << endl;
-            OPstream toSlave(Pstream::commsTypes::blocking, slave);
-            toSlave << data;
+            Perr<< "master sending to proc:" << proci << endl;
+            OPstream::bsend(tok, proci);
         }
     }
     else
     {
-        {
-            Perr<< "slave sending to master " << Pstream::masterNo() << endl;
-            OPstream toMaster
-            (
-                Pstream::commsTypes::blocking,
-                Pstream::masterNo()
-            );
+        Perr<< "proc sending to master" << endl;
+        OPstream::bsend(tok, UPstream::masterNo());
 
-            toMaster << data;
-        }
+        Perr<< "proc receiving from master" << endl;
+        IPstream::recv(tok, UPstream::masterNo());
 
-        Perr<< "slave receiving from master " << Pstream::masterNo() << endl;
-        IPstream fromMaster
-        (
-            Pstream::commsTypes::blocking,
-            Pstream::masterNo()
-        );
-
-        fromMaster >> tok;
         Perr<< tok.info() << endl;
     }
 }
@@ -212,12 +185,14 @@ void testTokenized(const T& data)
 
 int main(int argc, char *argv[])
 {
+    argList::noCheckProcessorDirectories();
+
     #include "setRootCase.H"
     #include "createTime.H"
 
     testMapDistribute();
 
-    if (!Pstream::parRun())
+    if (!UPstream::parRun())
     {
         Info<< "\nWarning: not parallel - skipping further tests\n" << endl;
         return 0;
diff --git a/applications/test/treeComms/Test-treeComms.C b/applications/test/treeComms/Test-treeComms.C
index 94588bd797..d31cc69694 100644
--- a/applications/test/treeComms/Test-treeComms.C
+++ b/applications/test/treeComms/Test-treeComms.C
@@ -51,7 +51,7 @@ void printConnection(Ostream& os, const label proci, const labelUList& below)
 // The number of receives - as per gatherList (v2112)
 void printRecvCount_gatherList
 (
-    const List<UPstream::commsStruct>& comms,
+    const UList<UPstream::commsStruct>& comms,
     const label comm = UPstream::worldComm
 )
 {
@@ -91,7 +91,7 @@ void printRecvCount_gatherList
 // The number of sends - as per scatterList (v2112)
 void printSendCount_scatterList
 (
-    const List<UPstream::commsStruct>& comms,
+    const UList<UPstream::commsStruct>& comms,
     const label comm = UPstream::worldComm
 )
 {
@@ -131,7 +131,7 @@ void printSendCount_scatterList
 // Transmission widths (contiguous data)
 void printWidths
 (
-    const List<UPstream::commsStruct>& comms,
+    const UList<UPstream::commsStruct>& comms,
     const label comm = UPstream::worldComm
 )
 {
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
index 5dda9b6e6e..87844b933e 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2013 OpenFOAM Foundation
-    Copyright (C) 2021-2023 OpenCFD Ltd.
+    Copyright (C) 2021-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -71,6 +71,33 @@ public:
             const label comm = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
+
+
+    // Static Functions
+
+        //- Receive and deserialize a value.
+        //- Uses \c operator>> for de-serialization
+        template<class Type>
+        static void recv
+        (
+            Type& value,
+            const int fromProcNo,
+            const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
+        )
+        {
+            IPstream is
+            (
+                UPstream::commsTypes::scheduled,
+                fromProcNo,
+                0,  // bufSize
+                tag,
+                comm,
+                fmt
+            );
+            is >> value;
+        }
 };
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
index 154ed87d31..bd1f6953e3 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2013 OpenFOAM Foundation
-    Copyright (C) 2021-2023 OpenCFD Ltd.
+    Copyright (C) 2021-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -71,6 +71,73 @@ public:
             const label comm = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
+
+
+    // Static Functions
+
+        //- Serialize a value and send (buffered/blocking or standard mode).
+        //- Uses \c operator<< for serialization
+        template<class Type>
+        static void send
+        (
+            const Type& value,
+            //! blocking or scheduled only!
+            const UPstream::commsTypes commsType,
+            const int toProcNo,
+            const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
+        )
+        {
+            OPstream os(commsType, toProcNo, 0, tag, comm, fmt);
+            os << value;
+        }
+
+        //- Serialize a value and send (buffered/blocking mode).
+        //- Uses \c operator<< for serialization
+        template<class Type>
+        static void bsend
+        (
+            const Type& value,
+            const int toProcNo,
+            const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
+        )
+        {
+            OPstream::send
+            (
+                value,
+                UPstream::commsTypes::blocking,
+                toProcNo,
+                tag,
+                comm,
+                fmt
+            );
+        }
+
+        //- Serialize a value and send (standard mode).
+        //- Uses \c operator<< for serialization
+        template<class Type>
+        static void send
+        (
+            const Type& value,
+            const int toProcNo,
+            const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm,
+            IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
+        )
+        {
+            OPstream::send
+            (
+                value,
+                UPstream::commsTypes::scheduled,
+                toProcNo,
+                tag,
+                comm,
+                fmt
+            );
+        }
 };
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
index fe251cbf91..04e3009503 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2016-2023 OpenCFD Ltd.
+    Copyright (C) 2016-2024 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -170,7 +170,7 @@ public:
 
     // Gather/combine data
     // Inplace combine values from processors.
-    // (Uses construct from Istream instead of <<)
+    // (Uses construct from Istream instead of \c << operator)
 
         //- Gather data, applying \c cop to inplace combine \c value
         //- from different processors.
@@ -178,6 +178,7 @@ public:
         template<class T, class CombineOp>
         static void combineGather
         (
+            //! [in,out]
             T& value,
             const CombineOp& cop,
             const int tag = UPstream::msgType(),
@@ -193,6 +194,7 @@ public:
         template<class T, class CombineOp>
         static void combineReduce
         (
+            //! [in,out]
             T& value,
             const CombineOp& cop,
             const int tag = UPstream::msgType(),
@@ -220,7 +222,8 @@ public:
         template<class T, class CombineOp>
         static void listCombineGather
         (
-            List<T>& values,
+            //! [in,out]
+            UList<T>& values,
             const CombineOp& cop,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm
@@ -232,6 +235,7 @@ public:
         template<class T, class CombineOp>
         static void listCombineReduce
         (
+            //! [in,out] - List (not UList) due to broadcast()
             List<T>& values,
             const CombineOp& cop,
             const int tag = UPstream::msgType(),
@@ -242,6 +246,7 @@ public:
         template<class T, class CombineOp>
         static void listCombineAllGather
         (
+            //! [in,out] - List (not UList) due to broadcast()
             List<T>& values,
             const CombineOp& cop,
             const int tag = UPstream::msgType(),
@@ -305,8 +310,9 @@ public:
         template<class T>
         static void gatherList
         (
-            const List<commsStruct>& comms,
-            List<T>& values,
+            const UList<commsStruct>& comms,
+            //! [in,out]
+            UList<T>& values,
             const int tag,
             const label comm
         );
@@ -316,7 +322,8 @@ public:
         template<class T>
         static void gatherList
         (
-            List<T>& values,
+            //! [in,out]
+            UList<T>& values,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm
         );
@@ -328,73 +335,35 @@ public:
         template<class T>
         static void allGatherList
         (
-            List<T>& values,
+            //! [in,out]
+            UList<T>& values,
             const int tag = UPstream::msgType(),
             const label comm = UPstream::worldComm
         );
 
 
-        // Scatter
+    // Scatter
 
-            //- Broadcast data
-            template<class T>
-            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
-            static void scatter
-            (
-                T& value,
-                const int tag = UPstream::msgType(),  //!< ignored
-                const label comm = UPstream::worldComm
-            );
+        //- Inverse of gatherList.
+        //- Uses the specified communication schedule.
+        template<class T>
+        static void scatterList
+        (
+            const UList<commsStruct>& comms,
+            UList<T>& values,
+            const int tag,
+            const label comm
+        );
 
-            //- Broadcast data
-            template<class T>
-            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
-            static void combineScatter
-            (
-                T& value,
-                const int tag = UPstream::msgType(),  //!< ignored
-                const label comm = UPstream::worldComm
-            );
-
-            //- Broadcast data
-            template<class T>
-            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
-            static void listCombineScatter
-            (
-                List<T>& value,
-                const int tag = UPstream::msgType(),  //!< ignored
-                const label comm = UPstream::worldComm
-            );
-
-            //- Broadcast data
-            template<class Container>
-            FOAM_DEPRECATED_FOR(2024-01, "broadcast()")
-            static void mapCombineScatter
-            (
-                Container& values,
-                const int tag = UPstream::msgType(),  //!< ignored
-                const label comm = UPstream::worldComm
-            );
-
-
-            //- Scatter data. Reverse of gatherList
-            template<class T>
-            static void scatterList
-            (
-                const List<commsStruct>& comms,
-                List<T>& values,
-                const int tag,
-                const label comm
-            );
-
-            //- Like above but switches between linear/tree communication
-            template<class T>
-            static void scatterList
-            (
-                List<T>& values,
-                const int tag = UPstream::msgType(),
-                const label comm = UPstream::worldComm
-            );
+        //- Inverse of gatherList.
+        //- Uses linear/tree communication.
+        template<class T>
+        static void scatterList
+        (
+            UList<T>& values,
+            const int tag = UPstream::msgType(),
+            const label comm = UPstream::worldComm
+        );
 
 
     // Exchange
@@ -573,6 +542,61 @@ public:
             const label comm,
             const bool wait = true  //!< (ignored)
         );
+
+
+    // Housekeeping
+
+        //- \deprecated(2024-01) Broadcast data
+        template<class T>
+        FOAM_DEPRECATED_FOR(2024-01, "Pstream::broadcast()")
+        static void scatter
+        (
+            T& value,
+            const int tag = UPstream::msgType(),  //!< ignored
+            const label comm = UPstream::worldComm
+        )
+        {
+            Pstream::broadcast(value, comm);
+        }
+
+        //- \deprecated(2024-01) Broadcast data
+        template<class T>
+        FOAM_DEPRECATED_FOR(2024-01, "Pstream::broadcast()")
+        static void combineScatter
+        (
+            T& value,
+            const int tag = UPstream::msgType(),  //!< ignored
+            const label comm = UPstream::worldComm
+        )
+        {
+            Pstream::broadcast(value, comm);
+        }
+
+        //- \deprecated(2024-01) Broadcast data
+        template<class T>
+        FOAM_DEPRECATED_FOR(2024-01, "Pstream::broadcast()")
+        static void listCombineScatter
+        (
+            List<T>& value,
+            const int tag = UPstream::msgType(),  //!< ignored
+            const label comm = UPstream::worldComm
+        )
+        {
+            Pstream::broadcast(value, comm);
+        }
+
+        //- \deprecated(2024-01) Broadcast data
+        template<class Container>
+        FOAM_DEPRECATED_FOR(2024-01, "Pstream::broadcast()")
+        static void mapCombineScatter
+        (
+            Container& values,
+            const int tag = UPstream::msgType(),  //!< ignored
+            const label comm = UPstream::worldComm
+        )
+        {
+            Pstream::broadcast(values, comm);
+        }
 };
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
index e9e2ce98bc..7bba29d4fb 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
@@ -148,7 +148,11 @@ namespace Foam
 
 //- Return a broadcasted value (uses a copy internally)
 template<class Type>
-Type returnBroadcast(const Type& value, const label comm)
+Type returnBroadcast
+(
+    const Type& value,
+    const label comm = UPstream::worldComm
+)
 {
     Type work(value);
     Pstream::broadcast(work, comm);
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
index 4696d00592..6b280517e0 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
@@ -107,7 +107,7 @@ void Foam::Pstream::combineGather
         }
 
         // Send up value
-        if (myComm.above() != -1)
+        if (myComm.above() >= 0)
         {
             if (debug & 2)
             {
@@ -166,7 +166,7 @@ void Foam::Pstream::combineReduce
 template<class T, class CombineOp>
 void Foam::Pstream::listCombineGather
 (
-    List<T>& values,
+    UList<T>& values,
     const CombineOp& cop,
     const int tag,
     const label comm
@@ -233,7 +233,7 @@ void Foam::Pstream::listCombineGather
         }
 
         // Send up values
-        if (myComm.above() != -1)
+        if (myComm.above() >= 0)
         {
             if (debug & 2)
             {
@@ -349,7 +349,7 @@ void Foam::Pstream::mapCombineGather
         }
 
         // Send up values
-        if (myComm.above() != -1)
+        if (myComm.above() >= 0)
         {
             if (debug & 2)
             {
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
index f879f91283..920a9a2216 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
@@ -26,7 +26,7 @@ License
 
 Description
     Gather data from all processors onto single processor according to some
-    communication schedule (usually linear-to-master or tree-to-master).
+    communication schedule (usually tree-to-master).
     The gathered data will be a single value constructed from the values
     on individual processors using a user-specified operator.
 
@@ -88,7 +88,7 @@ void Foam::Pstream::gather
         }
 
         // Send up value
-        if (myComm.above() != -1)
+        if (myComm.above() >= 0)
         {
             if (is_contiguous<T>::value)
             {
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
index d009e43eb8..74af324b34 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
@@ -26,7 +26,7 @@ License
 
 Description
     Gather data from all processors onto single processor according to some
-    communication schedule (usually linear-to-master or tree-to-master).
+    communication schedule (usually tree-to-master).
     The gathered data will be a list with element procID the data from processor
     procID. Before calling every processor should insert its value into
     values[UPstream::myProcNo(comm)].
@@ -45,24 +45,27 @@ Description
 template<class T>
 void Foam::Pstream::gatherList
 (
-    const List<UPstream::commsStruct>& comms,
-    List<T>& values,
+    const UList<UPstream::commsStruct>& comms,
+    UList<T>& values,
     const int tag,
     const label comm
 )
 {
     if (!comms.empty() && UPstream::is_parallel(comm))
     {
-        if (values.size() < UPstream::nProcs(comm))
+        const label myProci = UPstream::myProcNo(comm);
+        const label numProc = UPstream::nProcs(comm);
+
+        if (values.size() < numProc)
         {
             FatalErrorInFunction
-                << "List of values is too small:" << values.size()
-                << " vs numProcs:" << UPstream::nProcs(comm) << nl
+                << "List of values:" << values.size()
+                << " < numProcs:" << numProc << nl
                 << Foam::abort(FatalError);
         }
 
         // My communication order
-        const auto& myComm = comms[UPstream::myProcNo(comm)];
+        const auto& myComm = comms[myProci];
 
         // Receive from my downstairs neighbours
         for (const label belowID : myComm.below())
@@ -127,21 +130,21 @@ void Foam::Pstream::gatherList
         // Send up from values:
         // - my own value first
         // - all belowLeaves next
-        if (myComm.above() != -1)
+        if (myComm.above() >= 0)
         {
             const labelList& belowLeaves = myComm.allBelow();
 
             if (debug & 2)
             {
                 Pout<< " sending to " << myComm.above()
-                    << " data from me:" << UPstream::myProcNo(comm)
-                    << " data:" << values[UPstream::myProcNo(comm)] << endl;
+                    << " data from me:" << myProci
+                    << " data:" << values[myProci] << endl;
             }
 
             if (is_contiguous<T>::value)
             {
                 List<T> sending(belowLeaves.size() + 1);
-                sending[0] = values[UPstream::myProcNo(comm)];
+                sending[0] = values[myProci];
 
                 forAll(belowLeaves, leafI)
                 {
@@ -168,7 +171,7 @@ void Foam::Pstream::gatherList
                     tag,
                     comm
                 );
-                toAbove << values[UPstream::myProcNo(comm)];
+                toAbove << values[myProci];
 
                 for (const label leafID : belowLeaves)
                 {
@@ -189,8 +192,8 @@ void Foam::Pstream::gatherList
 template<class T>
 void Foam::Pstream::scatterList
 (
-    const List<UPstream::commsStruct>& comms,
-    List<T>& values,
+    const UList<UPstream::commsStruct>& comms,
+    UList<T>& values,
     const int tag,
     const label comm
 )
@@ -201,19 +204,22 @@ void Foam::Pstream::scatterList
 
     if (!comms.empty() && UPstream::is_parallel(comm))
     {
-        if (values.size() < UPstream::nProcs(comm))
+        const label myProci = UPstream::myProcNo(comm);
+        const label numProc = UPstream::nProcs(comm);
+
+        if (values.size() < numProc)
         {
             FatalErrorInFunction
-                << "List of values is too small:" << values.size()
-                << " vs numProcs:" << UPstream::nProcs(comm) << nl
+                << "List of values:" << values.size()
+                << " < numProcs:" << numProc << nl
                 << Foam::abort(FatalError);
         }
 
         // My communication order
-        const auto& myComm = comms[UPstream::myProcNo(comm)];
+        const auto& myComm = comms[myProci];
 
         // Receive from up
-        if (myComm.above() != -1)
+        if (myComm.above() >= 0)
         {
             const labelList& notBelowLeaves = myComm.allNotBelow();
 
@@ -318,7 +324,7 @@ void Foam::Pstream::scatterList
 template<class T>
 void Foam::Pstream::gatherList
 (
-    List<T>& values,
+    UList<T>& values,
     const int tag,
     const label comm
 )
@@ -337,7 +343,7 @@ void Foam::Pstream::gatherList
 template<class T>
 void Foam::Pstream::scatterList
 (
-    List<T>& values,
+    UList<T>& values,
     const int tag,
     const label comm
 )
@@ -355,7 +361,7 @@ void Foam::Pstream::scatterList
 template<class T>
 void Foam::Pstream::allGatherList
 (
-    List<T>& values,
+    UList<T>& values,
     const int tag,
     const label comm
 )
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 83760dfc3e..de61d0e46a 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -1202,6 +1202,7 @@ public:
 
         //- Process index of first sub-process
         //  \deprecated(2020-09) use subProcs() method instead
+        FOAM_DEPRECATED_FOR(2020-09, "subProcs() method")
         static constexpr int firstSlave() noexcept
         {
             return 1;
@@ -1209,6 +1210,7 @@ public:
 
         //- Process index of last sub-process
         //  \deprecated(2020-09) use subProcs() method instead
+        FOAM_DEPRECATED_FOR(2020-09, "subProcs() or allProcs() method")
         static int lastSlave(const label communicator = worldComm)
         {
             return nProcs(communicator) - 1;
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndex.C b/src/OpenFOAM/parallel/globalIndex/globalIndex.C
index 8f314ba328..218869d33b 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndex.C
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndex.C
@@ -290,7 +290,7 @@ Foam::globalIndex::bin
     if (!globalIds.empty())
     {
         labelList& binOffsets = bins.offsets();
-        binOffsets.resize(offsets.size(), Zero);
+        binOffsets.resize(offsets.size(), Foam::zero{});
 
         labelList& binValues = bins.values();
         binValues = UIndirectList<label>(globalIds, order);
@@ -372,7 +372,7 @@ void Foam::globalIndex::reset
             // TBD: check for (proci >= 0) ?
             const auto proci = UPstream::myProcNo(comm);
 
-            counts.resize(len, Zero);
+            counts.resize(len, Foam::zero{});
             counts[proci] = localSize;
         }
 
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndex.H b/src/OpenFOAM/parallel/globalIndex/globalIndex.H
index faad801f7d..f5b82a44cf 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndex.H
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndex.H
@@ -83,6 +83,14 @@ class globalIndex
             DynamicList<label>& validBins
         );
 
+        // Cannot use non-blocking for non-contiguous data.
+        // template<class Type>
+        // inline static UPstream::commsTypes getCommsType
+        // (
+        //     const UPstream::commsTypes preferred
+        //   = UPstream::commsTypes::nonBlocking
+        // );
+
         //- Report overflow at specified (non-negative) index
         static void reportOverflowAndExit
         (
@@ -187,7 +195,7 @@ public:
         inline label length() const noexcept;
 
         //- Global sum of localSizes. Same as totalSize()
-        FOAM_DEPRECATED_STRICT(2022-10, "totalSize() - less ambiguous")
+        FOAM_DEPRECATED_STRICT(2022-10, "totalSize() - unambiguous")
         inline label size() const;
 
         //- The span size covered by the offsets, zero if empty
@@ -722,10 +730,11 @@ public:
         //- Inplace collect data in processor order on master
         //- (in serial: a no-op).
         //  Communication with default/specified communicator, message tag.
-        //  After the gather, the field is zero-sized on the slaves.
+        //  After the gather, the field is zero-sized on non-master.
         template<class Type>
         void gatherInplace
         (
+            //! [in,out]
             List<Type>& fld,
             const int tag = UPstream::msgType(),
             const UPstream::commsTypes = UPstream::commsTypes::nonBlocking,
@@ -737,12 +746,11 @@ public:
         //  Communication with default/specified communicator.
         //  \attention The nProcs for globalIndex and communicator
         //      must match!!
-        //
-        //  The allData is output (master), zero-sized on non-master
         template<class Type, class OutputContainer = List<Type>>
         void mpiGather
         (
             const UList<Type>& sendData,
+            //! [out] output on master, zero-sized on non-master
             OutputContainer& allData,
             const label comm = UPstream::worldComm,  //!< communicator
 
@@ -780,6 +788,7 @@ public:
         template<class Type>
         void mpiGatherInplace
         (
+            //! [in,out]
             List<Type>& fld,
             const label comm = UPstream::worldComm,  //!< communicator
 
@@ -833,6 +842,7 @@ public:
         template<class Type>
         static void mpiGatherInplaceOp
         (
+            //! [in,out]
             List<Type>& fld,
             const label comm = UPstream::worldComm,  //!< communicator
 
@@ -844,12 +854,11 @@ public:
         //- Collect data in processor order on master
         //- (in serial: performs a simple copy).
         //  Communication with default/specified communicator, message tag.
-        //
-        //  The allFld is output (master), zero-sized on non-master
         template<class Type>
         static void gatherOp
         (
             const UList<Type>& sendData,
+            //! [out] output on master, zero-sized on non-master
             List<Type>& allData,
             const int tag = UPstream::msgType(),
             const UPstream::commsTypes = UPstream::commsTypes::nonBlocking,
@@ -859,12 +868,11 @@ public:
         //- Collect data in processor order on master
         //- (in serial: performs a simple copy).
         //  Communication with default/specified communicator, message tag.
-        //
-        //  The allFld is output (master), zero-sized on non-master
         template<class Type, class Addr>
         static void gatherOp
         (
             const IndirectListBase<Type, Addr>& sendData,
+            //! [out] output on master, zero-sized on non-master
             List<Type>& allData,
             const int tag = UPstream::msgType(),
             const UPstream::commsTypes = UPstream::commsTypes::nonBlocking,
@@ -903,10 +911,11 @@ public:
         //- (in serial: a no-op).
         //  Communication with default/specified communicator, message tag.
         //
-        //  After the gather, the field is zero-sized on the slaves.
+        //  After the gather, the field is zero-sized on non-master.
         template<class Type>
         static void gatherInplaceOp
         (
+            //! [in,out]
             List<Type>& fld,
             const int tag = UPstream::msgType(),
             const UPstream::commsTypes = UPstream::commsTypes::nonBlocking,
diff --git a/src/OpenFOAM/parallel/globalIndex/globalIndexTemplates.C b/src/OpenFOAM/parallel/globalIndex/globalIndexTemplates.C
index 3898913fe9..71c6672754 100644
--- a/src/OpenFOAM/parallel/globalIndex/globalIndexTemplates.C
+++ b/src/OpenFOAM/parallel/globalIndex/globalIndexTemplates.C
@@ -30,6 +30,25 @@ License
 
 // * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
 
+// Cannot use non-blocking for non-contiguous data.
+// template<class Type>
+// inline Foam::UPstream::commsTypes getCommsType
+// (
+//     const UPstream::commsTypes preferred
+// )
+// {
+//     return
+//     (
+//         (
+//             !is_contiguous<Type>::value
+//          && UPstream::commsTypes::nonBlocking == preferred
+//         )
+//       ? UPstream::commsTypes::scheduled
+//       : preferred
+//     );
+// }
+
+
 template<class Addr>
 Foam::labelList
 Foam::globalIndex::calcOffsets
@@ -115,8 +134,7 @@ void Foam::globalIndex::gatherValues
 {
     // low-level: no parRun guard
 
-    // Automatically change from nonBlocking to scheduled for
-    // non-contiguous data.
+    // Cannot use non-blocking for non-contiguous data.
     const UPstream::commsTypes commsType =
     (
         (
@@ -202,8 +220,7 @@ void Foam::globalIndex::gather
 {
     // low-level: no parRun guard
 
-    // Automatically change from nonBlocking to scheduled for
-    // non-contiguous data.
+    // Cannot use non-blocking for non-contiguous data.
     const UPstream::commsTypes commsType =
     (
         (
@@ -322,8 +339,7 @@ void Foam::globalIndex::gather
         return;
     }
 
-    // Automatically change from nonBlocking to scheduled for
-    // non-contiguous data.
+    // Cannot use non-blocking for non-contiguous data.
     const UPstream::commsTypes commsType =
     (
         (
@@ -907,8 +923,7 @@ void Foam::globalIndex::scatter
 {
     // low-level: no parRun guard
 
-    // Automatically change from nonBlocking to scheduled for
-    // non-contiguous data.
+    // Cannot use non-blocking for non-contiguous data.
     const UPstream::commsTypes commsType =
     (
         (