diff --git a/applications/test/parallel-comm2/Test-parallel-comm2.C b/applications/test/parallel-comm2/Test-parallel-comm2.C
index 53b8fc3e5e..7f5a4afa5a 100644
--- a/applications/test/parallel-comm2/Test-parallel-comm2.C
+++ b/applications/test/parallel-comm2/Test-parallel-comm2.C
@@ -52,8 +52,8 @@ void rankInfo(const label comm)
         << "(parent:" << UPstream::parent(comm) << ')'
         << " rank:" << ranki
         << "(sub:" << UPstream::is_subrank(comm)
-        << ") nProcs:" << UPstream::nProcs(comm)
-        << " baseProcNo:" << UPstream::baseProcNo(comm, ranki);
+        << ") nProcs:" << UPstream::nProcs(comm);
+      // << " baseProcNo:" << UPstream::baseProcNo(comm, ranki);
 }
 
 
@@ -82,8 +82,10 @@ int main(int argc, char *argv[])
     #include "setRootCase.H"
 
     Info<< nl
+        << "parallel:" << UPstream::parRun()
         << "nProcs = " << UPstream::nProcs()
-        << " with " << UPstream::nComms() << " predefined comm(s)" << nl;
+        << " with " << UPstream::nComms() << " predefined comm(s)."
+        << " proc:" << UPstream::myProcNo() << nl;
 
     Info<< nl;
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
index 5532260412..98c2486337 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBroadcast.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2022 OpenCFD Ltd.
+    Copyright (C) 2022-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -34,31 +34,28 @@ License
 template<class Type>
 void Foam::Pstream::broadcast(Type& value, const label comm)
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (is_contiguous<Type>::value)
     {
-        if (is_contiguous<Type>::value)
+        // Note: contains parallel guard internally
+        UPstream::broadcast
+        (
+            reinterpret_cast<char*>(&value),
+            sizeof(Type),
+            comm,
+            UPstream::masterNo()
+        );
+    }
+    else if (UPstream::is_parallel(comm))
+    {
+        if (UPstream::master(comm))
         {
-            // Note: contains parallel guard internally as well
-            UPstream::broadcast
-            (
-                reinterpret_cast<char*>(&value),
-                sizeof(Type),
-                comm,
-                UPstream::masterNo()
-            );
+            OPBstream os(UPstream::masterNo(), comm);
+            os << value;
         }
-        else
+        else  // UPstream::is_subrank(comm)
         {
-            if (UPstream::master(comm))
-            {
-                OPBstream os(UPstream::masterNo(), comm);
-                os << value;
-            }
-            else
-            {
-                IPBstream is(UPstream::masterNo(), comm);
-                is >> value;
-            }
+            IPBstream is(UPstream::masterNo(), comm);
+            is >> value;
         }
     }
 }
@@ -67,14 +64,14 @@ void Foam::Pstream::broadcast(Type& value, const label comm)
 template<class Type, class... Args>
 void Foam::Pstream::broadcasts(const label comm, Type& arg1, Args&&... args)
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         if (UPstream::master(comm))
         {
             OPBstream os(UPstream::masterNo(), comm);
             Detail::outputLoop(os, arg1, std::forward<Args>(args)...);
         }
-        else
+        else  // UPstream::is_subrank(comm)
         {
             IPBstream is(UPstream::masterNo(), comm);
             Detail::inputLoop(is, arg1, std::forward<Args>(args)...);
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
index 8460e33a73..41d427635b 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamCombineGather.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019-2022 OpenCFD Ltd.
+    Copyright (C) 2019-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -53,7 +53,7 @@ void Foam::Pstream::combineGather
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -155,7 +155,7 @@ void Foam::Pstream::combineScatter
     #ifndef Foam_Pstream_scatter_nobroadcast
     Pstream::broadcast(value, comm);
     #else
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -290,7 +290,7 @@ void Foam::Pstream::combineReduce
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         const auto& comms = UPstream::whichCommunication(comm);
 
@@ -312,7 +312,7 @@ void Foam::Pstream::listCombineGather
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -420,7 +420,7 @@ void Foam::Pstream::listCombineScatter
     #ifndef Foam_Pstream_scatter_nobroadcast
     Pstream::broadcast(values, comm);
     #else
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -540,7 +540,7 @@ void Foam::Pstream::listCombineReduce
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         const auto& comms = UPstream::whichCommunication(comm);
 
@@ -562,7 +562,7 @@ void Foam::Pstream::mapCombineGather
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -645,7 +645,7 @@ void Foam::Pstream::mapCombineScatter
     #ifndef Foam_Pstream_scatter_nobroadcast
     Pstream::broadcast(values, comm);
     #else
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const UPstream::commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -746,7 +746,7 @@ void Foam::Pstream::mapCombineReduce
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         const auto& comms = UPstream::whichCommunication(comm);
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C
index 6c590d76d8..ff6eeab4d4 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchange.C
@@ -469,6 +469,11 @@ void Foam::Pstream::exchange
 {
     static_assert(is_contiguous<Type>::value, "Contiguous data only!");
 
+    if (!UPstream::is_rank(comm))
+    {
+        return;  // Process not in communicator
+    }
+
     const label myProci = UPstream::myProcNo(comm);
     const label numProcs = UPstream::nProcs(comm);
 
@@ -482,7 +487,7 @@ void Foam::Pstream::exchange
 
     recvBufs.resize_nocopy(numProcs);
 
-    if (UPstream::parRun() && numProcs > 1)
+    if (UPstream::is_parallel(comm))
     {
         // Presize all receive buffers
         forAll(recvSizes, proci)
@@ -567,7 +572,7 @@ void Foam::Pstream::exchange
         }
     }
 
-    // Do myself
+    // Do myself. Already checked if in communicator
     recvBufs[myProci] = sendBufs[myProci];
 }
 
@@ -586,7 +591,6 @@ void Foam::Pstream::exchange
     static_assert(is_contiguous<Type>::value, "Contiguous data only!");
 
     const int myProci = UPstream::myProcNo(comm);
-    const label numProcs = UPstream::nProcs(comm);
 
     // Initial: clear out receive 'slots'
     // Preferrable to clear out the map entries instead of the map itself
@@ -598,7 +602,7 @@ void Foam::Pstream::exchange
         iter.val().clear();
     }
 
-    if (UPstream::parRun() && numProcs > 1)
+    if (UPstream::is_parallel(comm))
     {
         // Presize all receive buffers
         forAllIters(recvSizes, iter)
@@ -708,8 +712,8 @@ void Foam::Pstream::exchange
         }
     }
 
-
-    // Do myself
+    // Do myself (if actually in the communicator)
+    if (UPstream::is_rank(comm))
     {
         const auto iter = sendBufs.find(myProci);
 
@@ -746,6 +750,12 @@ void Foam::Pstream::exchangeSizes
     const label comm
 )
 {
+    if (!UPstream::is_rank(comm))
+    {
+        recvSizes.clear();
+        return;  // Process not in communicator
+    }
+
     const label myProci = UPstream::myProcNo(comm);
     const label numProcs = UPstream::nProcs(comm);
 
@@ -817,6 +827,12 @@ void Foam::Pstream::exchangeSizes
     const label comm
 )
 {
+    if (!UPstream::is_rank(comm))
+    {
+        recvSizes.clear();
+        return;  // Process not in communicator
+    }
+
     Pstream::exchangeSizes<Container>
     (
         neighProcs,  // send
@@ -842,6 +858,11 @@ void Foam::Pstream::exchangeSizes
     Map<label> sendSizes(2*sendBufs.size());
     recvSizes.clear();  // Done in allToAllConsensus too, but be explicit here
 
+    if (!UPstream::is_rank(comm))
+    {
+        return;  // Process not in communicator
+    }
+
     forAllConstIters(sendBufs, iter)
     {
         const label proci = iter.key();
@@ -871,6 +892,12 @@ void Foam::Pstream::exchangeSizes
     const label comm
 )
 {
+    if (!UPstream::is_rank(comm))
+    {
+        recvSizes.clear();
+        return;  // Process not in communicator
+    }
+
     const label numProcs = UPstream::nProcs(comm);
 
     if (sendBufs.size() != numProcs)
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C
index 5a9ff54349..cb529246a7 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamExchangeConsensus.C
@@ -80,6 +80,18 @@ void exchangeConsensus
     const label myProci = UPstream::myProcNo(comm);
     const label numProc = UPstream::nProcs(comm);
 
+    // Initial: clear all receive information
+    for (auto& buf : recvBufs)
+    {
+        buf.clear();
+    }
+    recvSizes = Zero;
+
+    if (!UPstream::is_rank(comm))
+    {
+        return;  // Process not in communicator
+    }
+
     // #ifdef FULLDEBUG
     if (sendBufs.size() > numProc)
     {
@@ -95,14 +107,7 @@ void exchangeConsensus
     }
     // #endif
 
-    // Initial: clear all receive information
-    for (auto& buf : recvBufs)
-    {
-        buf.clear();
-    }
-    recvSizes = Zero;
-
-    if (!UPstream::parRun() || numProc < 2)
+    if (!UPstream::is_parallel(comm))
     {
         // Do myself
         recvBufs[myProci] = sendBufs[myProci];
@@ -252,7 +257,6 @@ void exchangeConsensus
 
     const label startOfRequests = UPstream::nRequests();
     const label myProci = UPstream::myProcNo(comm);
-    const label numProc = UPstream::nProcs(comm);
 
     // Initial: clear out receive 'slots'
     // Preferrable to clear out the map entries instead of the map itself
@@ -264,7 +268,7 @@ void exchangeConsensus
         iter.val().clear();
     }
 
-    if (!UPstream::parRun() || numProc < 2)
+    if (!UPstream::is_parallel(comm))
     {
         // Do myself
         const auto iter = sendBufs.find(myProci);
@@ -293,10 +297,11 @@ void exchangeConsensus
         const auto& sendData = iter.val();
 
         #ifdef FULLDEBUG
-        if (proci >= numProc)
+        if (proci >= UPstream::nProcs(comm))
         {
             FatalErrorInFunction
-                << "Send buffer:" << proci << " >= numProcs:" << numProc
+                << "Send buffer:" << proci << " >= numProcs:"
+                << UPstream::nProcs(comm)
                 << Foam::abort(FatalError);
         }
         #endif
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
index 3317663ef8..daeb052876 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGather.C
@@ -48,7 +48,7 @@ void Foam::Pstream::gather
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
@@ -130,7 +130,7 @@ void Foam::Pstream::scatter
     #ifndef Foam_Pstream_scatter_nobroadcast
     Pstream::broadcast(value, comm);
     #else
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         // My communication order
         const commsStruct& myComm = comms[UPstream::myProcNo(comm)];
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
index 785f6aedf2..ef4a344692 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamGatherList.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2015-2022 OpenCFD Ltd.
+    Copyright (C) 2015-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,13 +51,12 @@ void Foam::Pstream::gatherList
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         if (values.size() != UPstream::nProcs(comm))
         {
             FatalErrorInFunction
-                << "Size of list:" << values.size()
-                << " does not equal the number of processors:"
+                << "Size of list:" << values.size() << " != numProcs:"
                 << UPstream::nProcs(comm)
                 << Foam::abort(FatalError);
         }
@@ -200,7 +199,7 @@ void Foam::Pstream::scatterList
     // between scatterList() and using broadcast(List<T>&) or a regular
     // scatter(List<T>&) is that processor-local data is skipped.
 
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
         if (values.size() != UPstream::nProcs(comm))
         {
@@ -349,8 +348,23 @@ void Foam::Pstream::allGatherList
     const label comm
 )
 {
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)
+    if (UPstream::is_parallel(comm))
     {
+        // TBD
+        // if (std::is_arithmetic<T>::value)   // or is_contiguous ?
+        // {
+        //     if (values.size() != UPstream::nProcs(comm))
+        //     {
+        //         FatalErrorInFunction
+        //             << "Size of list:" << values.size()
+        //             << " != number of processors:"
+        //             << UPstream::nProcs(comm)
+        //             << Foam::abort(FatalError);
+        //     }
+        //     UPstream::mpiAllGather(values.data_bytes(), sizeof(T), comm);
+        //     return;
+        // }
+
         const auto& comms = UPstream::whichCommunication(comm);
 
         Pstream::gatherList(comms, values, tag, comm);
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
index 0bcf41e9ca..ce96c9a9fd 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamReduceOps.H
@@ -79,7 +79,7 @@ void reduce
     const label comm = UPstream::worldComm
 )
 {
-    if (UPstream::parRun())
+    if (UPstream::is_parallel(comm))
     {
         Foam::reduce(UPstream::whichCommunication(comm), value, bop, tag, comm);
     }
@@ -375,7 +375,7 @@ void sumReduce
     const label comm = UPstream::worldComm
 )
 {
-    if (UPstream::parRun())
+    if (UPstream::is_parallel(comm))
     {
         Foam::reduce(value, sumOp<T>(), tag, comm);
         Foam::reduce(count, sumOp<label>(), tag, comm);
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index 6fc66f9021..e14bcfc066 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -142,12 +142,7 @@ void Foam::UPstream::setParRun(const label nProcs, const bool haveThreads)
 }
 
 
-Foam::label Foam::UPstream::allocateCommunicator
-(
-    const label parentIndex,
-    const labelUList& subRanks,
-    const bool doPstream
-)
+Foam::label Foam::UPstream::getAvailableCommIndex(const label parentIndex)
 {
     label index;
     if (!freeComms_.empty())
@@ -177,16 +172,29 @@ Foam::label Foam::UPstream::allocateCommunicator
         treeCommunication_.emplace_back();
     }
 
+    return index;
+}
+
+
+Foam::label Foam::UPstream::allocateCommunicator
+(
+    const label parentIndex,
+    const labelUList& subRanks,
+    const bool doPstream
+)
+{
+    const label index = getAvailableCommIndex(parentIndex);
+
     if (debug)
     {
-        Pout<< "Communicators : Allocating communicator " << index << endl
-            << "    parent : " << parentIndex << endl
-            << "    procs  : " << subRanks << endl
+        Pout<< "Allocating communicator " << index << nl
+            << "    parent : " << parentIndex << nl
+            << "    procs  : " << subRanks << nl
             << endl;
     }
 
-    // Initialise; overwritten by allocatePstreamCommunicator
-    myProcNo_[index] = 0;
+    // Initially treat as master, overwritten by allocatePstreamCommunicator
+    myProcNo_[index] = UPstream::masterNo();
 
     // The selected sub-ranks.
     // - transcribe from label to int. Treat negative values as 'ignore'
@@ -222,10 +230,6 @@ Foam::label Foam::UPstream::allocateCommunicator
 
     procIds.resize(numSubRanks);
 
-    // Size but do not fill structure - this is done on-the-fly
-    linearCommunication_[index] = List<commsStruct>(numSubRanks);
-    treeCommunication_[index] = List<commsStruct>(numSubRanks);
-
     if (doPstream && parRun())
     {
         allocatePstreamCommunicator(parentIndex, index);
@@ -243,16 +247,15 @@ Foam::label Foam::UPstream::allocateCommunicator
         ///         myProcNo_[index] = -(myProcNo_[parentIndex]+1);
         ///     }
         /// }
-
-        // Did communicator allocation adjust procIDs_ as well?
-        if (numSubRanks != procIDs_[index].size())
-        {
-            numSubRanks = procIDs_[index].size();
-            linearCommunication_[index] = List<commsStruct>(numSubRanks);
-            treeCommunication_[index] = List<commsStruct>(numSubRanks);
-        }
     }
 
+    // In case communicator allocation adjusted procIDs_
+    numSubRanks = procIDs_[index].size();
+
+    // Size but do not fill structure - this is done on-the-fly
+    linearCommunication_[index] = List<commsStruct>(numSubRanks);
+    treeCommunication_[index] = List<commsStruct>(numSubRanks);
+
     return index;
 }
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index 8fcd0ba7dc..5030e9cef0 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -90,10 +90,8 @@ public:
 
     // Public Classes
 
-        // Forward Declarations
-
         //- Wrapper for MPI_Request
-        class Request;
+        class Request;  // Forward Declaration
 
         //- Structure for communicating between processors
         class commsStruct
@@ -237,6 +235,9 @@ private:
         //- Set data for parallel running
         static void setParRun(const label nProcs, const bool haveThreads);
 
+        //- Initialise entries for new communicator. Return the index
+        static label getAvailableCommIndex(const label parentIndex);
+
         //- Allocate a communicator with index
         static void allocatePstreamCommunicator
         (
@@ -621,46 +622,57 @@ public:
 
         //- Test if this a parallel run
         //  Modify access is deprecated
-        static bool& parRun() noexcept
-        {
-            return parRun_;
-        }
+        static bool& parRun() noexcept { return parRun_; }
 
         //- Have support for threads
-        static bool haveThreads() noexcept
-        {
-            return haveThreads_;
-        }
+        static bool haveThreads() noexcept { return haveThreads_; }
 
-        //- Number of ranks in parallel run (for given communicator)
-        //- is 1 for serial run
+        //- Relative rank for the master process - is always 0.
+        static constexpr int masterNo() noexcept { return 0; }
+
+        //- Number of ranks in parallel run (for given communicator).
+        //- It is 1 for serial run
         static label nProcs(const label communicator = worldComm)
         {
             return procIDs_[communicator].size();
         }
 
-        //- Process index of the master (always 0)
-        static constexpr int masterNo() noexcept
+        //- Rank of this process in the communicator (starting from masterNo()).
+        //- Can be negative if the process is not a rank in the communicator
+        static int myProcNo(const label communicator = worldComm)
         {
-            return 0;
+            return myProcNo_[communicator];
         }
 
-        //- Am I the master rank
+        //- True if process corresponds to the master rank in the communicator
         static bool master(const label communicator = worldComm)
         {
             return myProcNo_[communicator] == masterNo();
         }
 
-        //- Is this process a sub-rank on the communicator
+        //- True if process corresponds to any rank (master or sub-rank)
+        //- in the given communicator
+        static bool is_rank(const label communicator = worldComm)
+        {
+            return myProcNo_[communicator] >= 0;
+        }
+
+        //- True if process corresponds to a sub-rank in the given communicator
         static bool is_subrank(const label communicator = worldComm)
         {
             return myProcNo_[communicator] > masterNo();
         }
 
-        //- Number of this process (starting from masterNo() = 0)
-        static int myProcNo(const label communicator = worldComm)
+        //- True if parallel algorithm or exchange is required.
+        //  This is when parRun() == true, the process corresponds to a rank
+        //  in the communicator and there is more than one rank in the
+        //  communicator
+        static bool is_parallel(const label communicator = worldComm)
         {
-            return myProcNo_[communicator];
+            return
+            (
+                parRun_ && is_rank(communicator) && nProcs(communicator) > 1
+            );
         }
 
         //- The parent communicator
@@ -669,7 +681,7 @@ public:
             return parentComm_(communicator);
         }
 
-        //- Process IDs within a given communicator
+        //- The list of ranks within a given communicator
         static List<int>& procID(const label communicator)
         {
             return procIDs_[communicator];
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
index 54a102f845..a6b99f6ce9 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamTemplates.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2021 OpenCFD Ltd.
+    Copyright (C) 2021-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -44,7 +44,10 @@ Foam::List<T> Foam::UPstream::listGatherValues
 
     List<T> allValues;
 
-    const label nproc = (UPstream::parRun() ? UPstream::nProcs(comm) : 1);
+    const label nproc =
+    (
+        UPstream::is_parallel(comm) ? UPstream::nProcs(comm) : 1
+    );
 
     if (nproc > 1)
     {
@@ -88,7 +91,10 @@ T Foam::UPstream::listScatterValues
     }
 
 
-    const label nproc = (UPstream::parRun() ? UPstream::nProcs(comm) : 1);
+    const label nproc =
+    (
+        UPstream::is_parallel(comm) ? UPstream::nProcs(comm) : 1
+    );
 
     T localValue;
 
@@ -115,13 +121,13 @@ T Foam::UPstream::listScatterValues
     {
         // non-parallel: return local value
 
-        if (allValues.empty())   // Extra safety
+        if (UPstream::is_rank(comm) && !allValues.empty())
         {
-            localValue = Zero;
+            localValue = allValues[0];
         }
         else
         {
-            localValue = allValues[0];
+            localValue = Zero;
         }
      }
 
diff --git a/src/Pstream/mpi/UPstream.C b/src/Pstream/mpi/UPstream.C
index 2be5913e71..576626ab51 100644
--- a/src/Pstream/mpi/UPstream.C
+++ b/src/Pstream/mpi/UPstream.C
@@ -696,9 +696,10 @@ void Foam::UPstream::freePstreamCommunicator(const label index)
 
 void Foam::UPstream::barrier(const label communicator, UPstream::Request* req)
 {
-    // No-op for non-parallel
-    if (!UPstream::parRun())
+    // No-op for non-parallel or not on communicator
+    if (!UPstream::parRun() || !UPstream::is_rank(communicator))
     {
+        PstreamGlobals::reset_request(req);
         return;
     }
 
@@ -753,7 +754,8 @@ Foam::UPstream::probeMessage
 {
     std::pair<int,int> result(-1, 0);
 
-    if (!UPstream::parRun())
+    // No-op for non-parallel or not on communicator
+    if (!UPstream::parRun() || !UPstream::is_rank(communicator))
     {
         return result;
     }
diff --git a/src/Pstream/mpi/UPstreamBroadcast.C b/src/Pstream/mpi/UPstreamBroadcast.C
index f5044970c7..522ce5032e 100644
--- a/src/Pstream/mpi/UPstreamBroadcast.C
+++ b/src/Pstream/mpi/UPstreamBroadcast.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2022 OpenCFD Ltd.
+    Copyright (C) 2022-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -39,7 +39,7 @@ bool Foam::UPstream::broadcast
     const int rootProcNo
 )
 {
-    if (!UPstream::parRun() || UPstream::nProcs(comm) < 2)
+    if (!UPstream::is_parallel(comm))
     {
         // Nothing to do - ignore
         return true;
diff --git a/src/Pstream/mpi/UPstreamReduce.C b/src/Pstream/mpi/UPstreamReduce.C
index 5314dd17ce..b9d526fd02 100644
--- a/src/Pstream/mpi/UPstreamReduce.C
+++ b/src/Pstream/mpi/UPstreamReduce.C
@@ -247,7 +247,7 @@ void Foam::sumReduce                                                          \
     const label comm                                                          \
 )                                                                             \
 {                                                                             \
-    if (UPstream::parRun() && UPstream::nProcs(comm) > 1)                     \
+    if (UPstream::is_parallel(comm))                                          \
     {                                                                         \
         Native values[2];                                                     \
         values[0] = static_cast<Native>(count);                               \
diff --git a/src/Pstream/mpi/UPstreamWrappingTemplates.C b/src/Pstream/mpi/UPstreamWrappingTemplates.C
index fba6933a17..53695b43b1 100644
--- a/src/Pstream/mpi/UPstreamWrappingTemplates.C
+++ b/src/Pstream/mpi/UPstreamWrappingTemplates.C
@@ -42,7 +42,7 @@ void Foam::PstreamDetail::broadcast0
     const label comm
 )
 {
-    if (!UPstream::parRun())
+    if (!UPstream::is_parallel(comm))
     {
         return;
     }
@@ -73,7 +73,7 @@ void Foam::PstreamDetail::reduce0
     const label comm
 )
 {
-    if (!UPstream::parRun())
+    if (!UPstream::is_parallel(comm))
     {
         return;
     }
@@ -127,13 +127,13 @@ void Foam::PstreamDetail::allReduce
 {
     PstreamGlobals::reset_request(req, requestID);
 
-    if (!UPstream::parRun())
+    const bool immediate = (req || requestID);
+
+    if (!UPstream::is_parallel(comm))
     {
         return;
     }
 
-    const bool immediate = (req || requestID);
-
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
     {
         if (immediate)
@@ -238,6 +238,11 @@ void Foam::PstreamDetail::allToAll
 
     const bool immediate = (req || requestID);
 
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+
     const label numProc = UPstream::nProcs(comm);
 
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
@@ -258,7 +263,10 @@ void Foam::PstreamDetail::allToAll
         error::printStack(Pout);
     }
 
-    if (sendData.size() != numProc || recvData.size() != numProc)
+    if
+    (
+        (sendData.size() != numProc || recvData.size() != numProc)
+    )
     {
         FatalErrorInFunction
             << "Have " << numProc << " ranks, but size of sendData:"
@@ -267,7 +275,7 @@ void Foam::PstreamDetail::allToAll
             << Foam::abort(FatalError);
     }
 
-    if (!UPstream::parRun() || numProc < 2)
+    if (!UPstream::is_parallel(comm))
     {
         recvData.deepCopy(sendData);
         return;
@@ -365,6 +373,11 @@ void Foam::PstreamDetail::allToAllv
 
     const bool immediate = (req || requestID);
 
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+
     const label np = UPstream::nProcs(comm);
 
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
@@ -400,7 +413,7 @@ void Foam::PstreamDetail::allToAllv
             << Foam::abort(FatalError);
     }
 
-    if (!UPstream::parRun())
+    if (!UPstream::is_parallel(comm))
     {
         if (recvCounts[0] != sendCounts[0])
         {
@@ -502,6 +515,11 @@ void Foam::PstreamDetail::allToAllConsensus
     const label comm
 )
 {
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+
     const label myProci = UPstream::myProcNo(comm);
     const label numProc = UPstream::nProcs(comm);
 
@@ -529,7 +547,7 @@ void Foam::PstreamDetail::allToAllConsensus
     const Type zeroValue = pTraits<Type>::zero;
     recvData = zeroValue;
 
-    if (!UPstream::parRun() || numProc < 2)
+    if (!UPstream::is_parallel(comm))
     {
         // deep copy
         recvData.deepCopy(sendData);
@@ -691,6 +709,11 @@ void Foam::PstreamDetail::allToAllConsensus
     const label myProci = UPstream::myProcNo(comm);
     const label numProc = UPstream::nProcs(comm);
 
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
     {
         Pout<< "** non-blocking consensus Alltoall (map):";
@@ -706,7 +729,7 @@ void Foam::PstreamDetail::allToAllConsensus
     const Type zeroValue = pTraits<Type>::zero;
     recvBufs.clear();
 
-    if (!UPstream::parRun() || numProc < 2)
+    if (!UPstream::is_parallel(comm))
     {
         // Do myself
         const auto iter = sendBufs.find(myProci);
@@ -873,15 +896,19 @@ void Foam::PstreamDetail::gather
 {
     PstreamGlobals::reset_request(req, requestID);
 
-    if (!UPstream::parRun())
+    const bool immediate = (req || requestID);
+
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+    if (!UPstream::is_parallel(comm))
     {
         std::memmove(recvData, sendData, recvCount*sizeof(Type));
         return;
     }
 
-    const bool immediate = (req || requestID);
-
-    const label np = UPstream::nProcs(comm);
+    const label numProc = UPstream::nProcs(comm);
 
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
     {
@@ -893,7 +920,7 @@ void Foam::PstreamDetail::gather
         {
             Pout<< "** MPI_Gather (blocking):";
         }
-        Pout<< " np:" << np
+        Pout<< " numProc:" << numProc
             << " recvCount:" << recvCount
             << " with comm:" << comm
             << " warnComm:" << UPstream::warnComm
@@ -989,15 +1016,19 @@ void Foam::PstreamDetail::scatter
 {
     PstreamGlobals::reset_request(req, requestID);
 
-    if (!UPstream::parRun())
+    const bool immediate = (req || requestID);
+
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+    if (!UPstream::is_parallel(comm))
     {
         std::memmove(recvData, sendData, recvCount*sizeof(Type));
         return;
     }
 
-    const bool immediate = (req || requestID);
-
-    const label np = UPstream::nProcs(comm);
+    const label numProc = UPstream::nProcs(comm);
 
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
     {
@@ -1009,7 +1040,7 @@ void Foam::PstreamDetail::scatter
         {
             Pout<< "** MPI_Scatter (blocking):";
         }
-        Pout<< " np:" << np
+        Pout<< " numProc:" << numProc
             << " recvCount:" << recvCount
             << " with comm:" << comm
             << " warnComm:" << UPstream::warnComm
@@ -1106,15 +1137,19 @@ void Foam::PstreamDetail::gatherv
 {
     PstreamGlobals::reset_request(req, requestID);
 
-    if (!UPstream::parRun())
+    const bool immediate = (req || requestID);
+
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+    if (!UPstream::is_parallel(comm))
     {
         // recvCounts[0] may be invalid - use sendCount instead
         std::memmove(recvData, sendData, sendCount*sizeof(Type));
         return;
     }
 
-    const bool immediate = (req || requestID);
-
     const label np = UPstream::nProcs(comm);
 
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
@@ -1249,14 +1284,18 @@ void Foam::PstreamDetail::scatterv
 {
     PstreamGlobals::reset_request(req, requestID);
 
-    if (!UPstream::parRun())
+    const bool immediate = (req || requestID);
+
+    if (!UPstream::is_rank(comm))
+    {
+        return;
+    }
+    if (!UPstream::is_parallel(comm))
     {
         std::memmove(recvData, sendData, recvCount*sizeof(Type));
         return;
     }
 
-    const bool immediate = (req || requestID);
-
     const label np = UPstream::nProcs(comm);
 
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
@@ -1383,14 +1422,12 @@ void Foam::PstreamDetail::allGather
 
     const bool immediate = (req || requestID);
 
-    if (!UPstream::parRun() || UPstream::nProcs(comm) < 2)
+    if (!UPstream::is_parallel(comm))
     {
         // Nothing to do - ignore
         return;
     }
 
-    const label numProc = UPstream::nProcs(comm);
-
     if (UPstream::warnComm >= 0 && comm != UPstream::warnComm)
     {
         if (immediate)
@@ -1401,7 +1438,7 @@ void Foam::PstreamDetail::allGather
         {
             Pout<< "** MPI_Allgather (blocking):";
         }
-        Pout<< " numProc:" << numProc
+        Pout<< " numProc:" << UPstream::nProcs(comm)
             << " with comm:" << comm
             << " warnComm:" << UPstream::warnComm
             << endl;