diff --git a/applications/test/globalIndex3/Test-globalIndex3.cxx b/applications/test/globalIndex3/Test-globalIndex3.cxx
index 19a72499d3..d6ae4a658e 100644
--- a/applications/test/globalIndex3/Test-globalIndex3.cxx
+++ b/applications/test/globalIndex3/Test-globalIndex3.cxx
@@ -508,7 +508,7 @@ int main(int argc, char *argv[])
 
     #include "setRootCase.H"
 
-    const bool useLocalComms = UPstream::usingNodeComms();
+    const bool useLocalComms = UPstream::usingNodeComms(UPstream::worldComm);
     bool useWindow = args.found("window");
     bool useBuiltin = args.found("builtin");
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
index 472353a2cd..056d3dd0fb 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstream.H
@@ -61,14 +61,13 @@ public:
     // Constructors
 
         //- Construct given process index to read from
-        //- and optional buffer size, read format
         IPstream
         (
             const UPstream::commsTypes commsType,
             const int fromProcNo,
-            const label bufSize = 0,
+            const int bufferSize = 0,  //!< optional buffer size
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 
@@ -83,7 +82,7 @@ public:
             Type& value,
             const int fromProcNo,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         )
         {
@@ -91,9 +90,9 @@ public:
             (
                 UPstream::commsTypes::scheduled,  // ie, MPI_Recv()
                 fromProcNo,
-                0,  // bufSize
+                0,  // bufferSize
                 tag,
-                comm,
+                communicator,
                 fmt
             );
             is >> value;
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C b/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C
index 340f0a2f99..667970803b 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/IPstreams.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2022-2023 OpenCFD Ltd.
+    Copyright (C) 2022-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -38,7 +38,7 @@ Foam::UIPstream::UIPstream
     DynamicList<char>& receiveBuf,
     label& receiveBufPosition,
     const int tag,
-    const label comm,
+    const int communicator,
     const bool clearAtEnd,
     IOstreamOption::streamFormat fmt
 )
@@ -50,7 +50,7 @@ Foam::UIPstream::UIPstream
         receiveBuf,
         receiveBufPosition,
         tag,
-        comm,
+        communicator,
         clearAtEnd,
         fmt
     )
@@ -105,13 +105,13 @@ Foam::IPstream::IPstream
 (
     const UPstream::commsTypes commsType,
     const int fromProcNo,
-    const label bufSize,
+    const int bufferSize,
     const int tag,
-    const label comm,
+    const int communicator,
     IOstreamOption::streamFormat fmt
 )
 :
-    Pstream(commsType, bufSize),
+    Pstream(commsType, bufferSize),
     UIPstream
     (
         commsType,
@@ -119,7 +119,7 @@ Foam::IPstream::IPstream
         Pstream::transferBuf_,
         UIPstreamBase::storedRecvBufPos_,   // Internal only
         tag,
-        comm,
+        communicator,
         false,  // Do not clear Pstream::transferBuf_ if at end
         fmt
     )
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
index dcdcbda797..8513b4fbf1 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H
@@ -61,14 +61,13 @@ public:
     // Constructors
 
         //- Construct for given process index to send to
-        //- and optional buffer size, write format
         OPstream
         (
             const UPstream::commsTypes commsType,
             const int toProcNo,
-            const label bufSize = 0,
+            const int bufferSize = 0,  //!< optional buffer size
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 
@@ -81,15 +80,15 @@ public:
         static void send
         (
             const Type& value,
-            //! blocking or scheduled only!
+            //! buffered or scheduled only!
             const UPstream::commsTypes commsType,
             const int toProcNo,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         )
         {
-            OPstream os(commsType, toProcNo, 0, tag, comm, fmt);
+            OPstream os(commsType, toProcNo, 0, tag, communicator, fmt);
             os << value;
         }
 
@@ -101,7 +100,7 @@ public:
             const Type& value,
             const int toProcNo,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         )
         {
@@ -111,7 +110,7 @@ public:
                 UPstream::commsTypes::scheduled,  // ie, MPI_Send()
                 toProcNo,
                 tag,
-                comm,
+                communicator,
                 fmt
             );
         }
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C b/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
index fc8312f674..281bc01a68 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/OPstreams.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011 OpenFOAM Foundation
-    Copyright (C) 2022-2024 OpenCFD Ltd.
+    Copyright (C) 2022-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -37,7 +37,7 @@ Foam::UOPstream::UOPstream
     const int toProcNo,
     DynamicList<char>& sendBuf,
     const int tag,
-    const label comm,
+    const int comm,
     const bool sendAtDestruct,
     IOstreamOption::streamFormat fmt
 )
@@ -66,20 +66,20 @@ Foam::OPstream::OPstream
 (
     const UPstream::commsTypes commsType,
     const int toProcNo,
-    const label bufSize,
+    const int bufferSize,
     const int tag,
-    const label comm,
+    const int communicator,
     IOstreamOption::streamFormat fmt
 )
 :
-    Pstream(commsType, bufSize),
+    Pstream(commsType, bufferSize),
     UOPstream
     (
         commsType,
         toProcNo,
         Pstream::transferBuf_,
         tag,
-        comm,
+        communicator,
         true,  // sendAtDestruct
         fmt
     )
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
index 84d97ee64f..ef8acb773b 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H
@@ -75,18 +75,20 @@ public:
 
     // Constructors
 
-        //- Construct for given communication type, with optional buffer size
-        explicit Pstream
-        (
-            const UPstream::commsTypes commsType,
-            const label bufSize = 0
-        )
+        //- Construct for communication type with empty buffer
+        explicit Pstream(const UPstream::commsTypes commsType) noexcept
+        :
+            UPstream(commsType)
+        {}
+
+        //- Construct for communication type with given buffer size
+        Pstream(const UPstream::commsTypes commsType, int bufferSize)
         :
             UPstream(commsType)
         {
-            if (bufSize > 0)
+            if (bufferSize > 0)
             {
-                transferBuf_.setCapacity(bufSize + 2*sizeof(scalar) + 1);
+                transferBuf_.setCapacity(bufferSize + 2*sizeof(scalar) + 1);
             }
         }
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
index 391c094bea..5332f3cf89 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2021-2023 OpenCFD Ltd.
+    Copyright (C) 2021-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -300,7 +300,7 @@ Foam::PstreamBuffers::PstreamBuffers
 (
     UPstream::commsTypes commsType,
     int tag,
-    label communicator,
+    int communicator,
     IOstreamOption::streamFormat fmt
 )
 :
@@ -313,7 +313,7 @@ Foam::PstreamBuffers::PstreamBuffers
     nProcs_(UPstream::nProcs(comm_)),
     sendBuffers_(nProcs_),
     recvBuffers_(nProcs_),
-    recvPositions_(nProcs_, Zero)
+    recvPositions_(nProcs_, Foam::zero{})
 {
     DebugPoutInFunction
         << "tag:" << tag_
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
index 97cb68fe9a..d80607d379 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2021-2023 OpenCFD Ltd.
+    Copyright (C) 2021-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -182,10 +182,10 @@ class PstreamBuffers
         const int tag_;
 
         //- Communicator
-        const label comm_;
+        const int comm_;
 
         //- Number of ranks associated with PstreamBuffers (at construction)
-        const label nProcs_;
+        const int nProcs_;
 
 
     // Buffer storage
@@ -266,7 +266,7 @@ public:
         (
             UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
             int tag = UPstream::msgType(),
-            label communicator = UPstream::worldComm,
+            int communicator = UPstream::worldComm,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
 
@@ -274,7 +274,7 @@ public:
         //- (default: nonBlocking), message tag, IO format (default: binary)
         explicit PstreamBuffers
         (
-            label communicator,
+            int communicator,
             UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
             int tag = UPstream::msgType(),
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
@@ -287,7 +287,7 @@ public:
         //- (default: nonBlocking), IO format (default: binary)
         PstreamBuffers
         (
-            label communicator,
+            int communicator,
             int tag,
             UPstream::commsTypes commsType = UPstream::commsTypes::nonBlocking,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
@@ -306,34 +306,19 @@ public:
     // Attributes
 
         //- The associated buffer format (ascii | binary)
-        IOstreamOption::streamFormat format() const noexcept
-        {
-            return format_;
-        }
+        IOstreamOption::streamFormat format() const noexcept { return format_; }
 
         //- The communications type of the stream
-        UPstream::commsTypes commsType() const noexcept
-        {
-            return commsType_;
-        }
+        UPstream::commsTypes commsType() const noexcept { return commsType_; }
 
         //- The transfer message tag
-        int tag() const noexcept
-        {
-            return tag_;
-        }
+        int tag() const noexcept { return tag_; }
 
         //- The communicator index
-        label comm() const noexcept
-        {
-            return comm_;
-        }
+        int comm() const noexcept { return comm_; }
 
         //- Number of ranks associated with PstreamBuffers
-        label nProcs() const noexcept
-        {
-            return nProcs_;
-        }
+        int nProcs() const noexcept { return nProcs_; }
 
 
     // Sizing
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
index dbce6dc010..33b40e8a22 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H
@@ -124,7 +124,7 @@ protected:
             DynamicList<char>& receiveBuf,
             label& receiveBufPosition,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             const bool clearAtEnd = false,   // destroy receiveBuf if at end
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
@@ -252,7 +252,7 @@ public:
             DynamicList<char>& receiveBuf,
             label& receiveBufPosition,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             const bool clearAtEnd = false,   // destroy receiveBuf if at end
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
@@ -292,7 +292,7 @@ public:
             char* buf,
             const std::streamsize bufSize,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             //! [out] request information (for non-blocking)
             UPstream::Request* req = nullptr
         );
@@ -307,7 +307,7 @@ public:
             char* buf,
             const std::streamsize bufSize,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm
+            const int communicator = UPstream::worldComm
         )
         {
             return UIPstream::read
@@ -317,7 +317,7 @@ public:
                 buf,
                 bufSize,
                 tag,
-                comm,
+                communicator,
                &req
             );
         }
@@ -332,7 +332,7 @@ public:
             const int fromProcNo,
             UList<Type>& buffer,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             //! [out] request information (for non-blocking)
             UPstream::Request* req = nullptr
         )
@@ -344,7 +344,7 @@ public:
                 buffer.data_bytes(),
                 buffer.size_bytes(),
                 tag,
-                comm,
+                communicator,
                 req
             );
         }
@@ -359,7 +359,7 @@ public:
             const int fromProcNo,
             SubList<Type> buffer,  // passed by shallow copy
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             //! [out] request information (for non-blocking)
             UPstream::Request* req = nullptr
         )
@@ -371,7 +371,7 @@ public:
                 buffer.data_bytes(),
                 buffer.size_bytes(),
                 tag,
-                comm,
+                communicator,
                 req
             );
         }
@@ -387,7 +387,7 @@ public:
             const int fromProcNo,
             UList<Type>& buffer,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm
+            const int communicator = UPstream::worldComm
         )
         {
             return UIPstream::read
@@ -397,7 +397,7 @@ public:
                 buffer.data_bytes(),
                 buffer.size_bytes(),
                 tag,
-                comm,
+                communicator,
                &req
             );
         }
@@ -413,7 +413,7 @@ public:
             const int fromProcNo,
             SubList<Type> buffer,  // passed by shallow copy
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm
+            const int communicator = UPstream::worldComm
         )
         {
             return UIPstream::read
@@ -423,7 +423,7 @@ public:
                 buffer.data_bytes(),
                 buffer.size_bytes(),
                 tag,
-                comm,
+                communicator,
                &req
             );
         }
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
index 9ea930111f..c9444e6735 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C
@@ -167,7 +167,7 @@ Foam::UIPstreamBase::UIPstreamBase
     DynamicList<char>& receiveBuf,
     label& receiveBufPosition,
     const int tag,
-    const label comm,
+    const int communicator,
     const bool clearAtEnd,
     IOstreamOption::streamFormat fmt
 )
@@ -176,7 +176,7 @@ Foam::UIPstreamBase::UIPstreamBase
     Istream(fmt),
     fromProcNo_(fromProcNo),
     tag_(tag),
-    comm_(comm),
+    comm_(communicator),
     messageSize_(0),
     storedRecvBufPos_(0),
     clearAtEnd_(clearAtEnd),
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
index 34c4968511..d1550c4d9c 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H
@@ -121,7 +121,7 @@ protected:
             const int toProcNo,
             DynamicList<char>& sendBuf,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             const bool sendAtDestruct = true,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
@@ -325,7 +325,7 @@ public:
             const int toProcNo,
             DynamicList<char>& sendBuf,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             const bool sendAtDestruct = true,
             IOstreamOption::streamFormat fmt = IOstreamOption::BINARY
         );
@@ -367,7 +367,7 @@ public:
             const char* buf,
             const std::streamsize bufSize,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             //! [out] request information (for non-blocking)
             UPstream::Request* req = nullptr,
             const UPstream::sendModes sendMode = UPstream::sendModes::normal
@@ -383,7 +383,7 @@ public:
             const char* buf,
             const std::streamsize bufSize,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             const UPstream::sendModes sendMode = UPstream::sendModes::normal
         )
         {
@@ -394,7 +394,7 @@ public:
                 buf,
                 bufSize,
                 tag,
-                comm,
+                communicator,
                &req,
                 sendMode
             );
@@ -410,7 +410,7 @@ public:
             const int toProcNo,
             const UList<Type>& buffer,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             //! [out] request information (for non-blocking)
             UPstream::Request* req = nullptr,
             const UPstream::sendModes sendMode = UPstream::sendModes::normal
@@ -423,7 +423,7 @@ public:
                 buffer.cdata_bytes(),
                 buffer.size_bytes(),
                 tag,
-                comm,
+                communicator,
                 req,
                 sendMode
             );
@@ -440,7 +440,7 @@ public:
             const int toProcNo,
             const UList<Type>& buffer,
             const int tag = UPstream::msgType(),
-            const label comm = UPstream::worldComm,
+            const int communicator = UPstream::worldComm,
             const UPstream::sendModes sendMode = UPstream::sendModes::normal
         )
         {
@@ -451,7 +451,7 @@ public:
                 buffer.cdata_bytes(),
                 buffer.size_bytes(),
                 tag,
-                comm,
+                communicator,
                &req,
                 sendMode
             );
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C
index e07c5320e8..cc665f0cb6 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UOPstreamBase.C
@@ -146,7 +146,7 @@ Foam::UOPstreamBase::UOPstreamBase
     const int toProcNo,
     DynamicList<char>& sendBuf,
     const int tag,
-    const label comm,
+    const int communicator,
     const bool sendAtDestruct,
     IOstreamOption::streamFormat fmt
 )
@@ -155,7 +155,7 @@ Foam::UOPstreamBase::UOPstreamBase
     Ostream(fmt),
     toProcNo_(toProcNo),
     tag_(tag),
-    comm_(comm),
+    comm_(communicator),
     sendAtDestruct_(sendAtDestruct),
     sendBuf_(sendBuf)
 {
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
index d3b8e489e3..2c1d72b143 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.C
@@ -112,7 +112,7 @@ void Foam::UPstream::printTopoControl(Ostream& os)
 
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
-void Foam::UPstream::setParRun(const label nProcs, const bool haveThreads)
+void Foam::UPstream::setParRun(const int nProcs, const bool haveThreads)
 {
     parRun_ = (nProcs > 0);
     haveThreads_ = haveThreads;
@@ -725,7 +725,7 @@ void Foam::UPstream::printCommTree
 }
 
 
-bool Foam::UPstream::usingNodeComms(const label communicator)
+bool Foam::UPstream::usingNodeComms(const int communicator)
 {
     // Starting point must be "real" world-communicator
     // ("real" means without any local trickery with worldComm)
@@ -849,10 +849,10 @@ Foam::DynamicList<Foam::UPstream::commsStructList>
 Foam::UPstream::treeCommunication_(16);
 
 
-Foam::label Foam::UPstream::constWorldComm_(0);
-Foam::label Foam::UPstream::numNodes_(1);
-Foam::label Foam::UPstream::commInterNode_(-1);
-Foam::label Foam::UPstream::commLocalNode_(-1);
+int Foam::UPstream::constWorldComm_(0);
+int Foam::UPstream::commInterNode_(-1);
+int Foam::UPstream::commLocalNode_(-1);
+int Foam::UPstream::numNodes_(1);
 
 Foam::label Foam::UPstream::worldComm(0);  // Initially same as constWorldComm_
 Foam::label Foam::UPstream::warnComm(-1);
@@ -860,7 +860,7 @@ Foam::label Foam::UPstream::warnComm(-1);
 
 // Predefine world and self communicator slots.
 // These are overwritten in parallel mode (by UPstream::setParRun())
-const Foam::label nPredefinedComm = []()
+const int nPredefinedComm = []()
 {
     // 0: COMM_WORLD : commGlobal(), constWorldComm_, worldComm
     (void) Foam::UPstream::newCommunicator(-1, Foam::labelRange(1), false);
@@ -894,7 +894,6 @@ registerOptSwitch
     Foam::UPstream::nodeCommsMin_
 );
 
-
 int Foam::UPstream::topologyControl_
 (
     Foam::debug::optimisationSwitch("topoControl", 0)
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
index a18d46128e..3ce948183f 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstream.H
@@ -389,18 +389,18 @@ private:
         //- Index to the world-communicator as defined at startup
         //- (after any multi-world definitions).
         //- Is unaffected by any later changes to worldComm.
-        static label constWorldComm_;
-
-        //- The number of shared/host nodes in the (const) world communicator.
-        static label numNodes_;
+        static int constWorldComm_;
 
         //- Index to the inter-node communicator (between nodes),
         //- defined based on constWorldComm_
-        static label commInterNode_;
+        static int commInterNode_;
 
         //- Index to the intra-host communicator (within a node),
         //- defined based on constWorldComm_
-        static label commLocalNode_;
+        static int commLocalNode_;
+
+        //- The number of shared/host nodes in the (const) world communicator.
+        static int numNodes_;
 
         //- Names of all worlds
         static wordList allWorlds_;
@@ -433,7 +433,7 @@ private:
     // Private Member Functions
 
         //- Set data for parallel running
-        static void setParRun(const label nProcs, const bool haveThreads);
+        static void setParRun(const int nProcs, const bool haveThreads);
 
         //- Initialise entries for new communicator.
         //
@@ -557,16 +557,16 @@ public:
 
         //- Communicator for all ranks, irrespective of any local worlds.
         //  This value \em never changes during a simulation.
-        static constexpr label commGlobal() noexcept { return 0; }
+        static constexpr int commGlobal() noexcept { return 0; }
 
         //- Communicator within the current rank only
         //  This value \em never changes during a simulation.
-        static constexpr label commSelf() noexcept { return 1; }
+        static constexpr int commSelf() noexcept { return 1; }
 
         //- Communicator for all ranks (respecting any local worlds).
         //  This value \em never changes after startup. Unlike the commWorld()
         //  which can be temporarily overriden.
-        static label commConstWorld() noexcept { return constWorldComm_; }
+        static int commConstWorld() noexcept { return constWorldComm_; }
 
         //- Communicator for all ranks (respecting any local worlds)
         static label commWorld() noexcept { return worldComm; }
@@ -601,13 +601,13 @@ public:
     // Host Communicators
 
         //- Communicator between nodes/hosts (respects any local worlds)
-        static label commInterNode() noexcept
+        static int commInterNode() noexcept
         {
             return (parRun_ ? commInterNode_ : constWorldComm_);
         }
 
         //- Communicator within the node/host (respects any local worlds)
-        static label commLocalNode() noexcept
+        static int commLocalNode() noexcept
         {
             return (parRun_ ? commLocalNode_ : constWorldComm_);
         }
@@ -626,7 +626,7 @@ public:
         //- it is running in parallel, the starting point is the
         //- world-communicator and it is not an odd corner case
         //- (ie, all processes on one node, all processes on different nodes)
-        static bool usingNodeComms(const label communicator = worldComm);
+        static bool usingNodeComms(const int communicator);
 
 
     // Constructors
@@ -1168,19 +1168,16 @@ public:
         }
 
         //- The number of shared/host nodes in the (const) world communicator.
-        static label numNodes() noexcept
-        {
-            return numNodes_;
-        }
+        static int numNodes() noexcept { return numNodes_; }
 
         //- The parent communicator
-        static label parent(const label communicator)
+        static label parent(int communicator)
         {
             return parentComm_(communicator);
         }
 
         //- The list of ranks within a given communicator
-        static List<int>& procID(const label communicator)
+        static List<int>& procID(int communicator)
         {
             return procIDs_[communicator];
         }
@@ -1694,7 +1691,7 @@ public:
         //      UPstream::Communicator::lookup(UPstream::commWorld())
         //  )
         //  \endcode
-        static Communicator lookup(const label comm);
+        static Communicator lookup(const int comm);
 
 
     // Member Functions
@@ -1713,6 +1710,9 @@ public:
 
         //- Reset to default constructed value (MPI_COMM_NULL)
         void reset() noexcept;
+
+        //- The number of ranks associated with the communicator
+        int size() const;
 };
 
 
diff --git a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamWindow.H b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamWindow.H
index aa3ef233f8..54ce90aecc 100644
--- a/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamWindow.H
+++ b/src/OpenFOAM/db/IOstreams/Pstreams/UPstreamWindow.H
@@ -137,6 +137,11 @@ public:
 
         //- Reset to default constructed value (MPI_WIN_NULL)
         void reset() noexcept;
+
+        //- The number of ranks associated with the window group.
+        //  The same as querying the original communicator, assuming the
+        //  communicator is available within the current code scope.
+        int size() const;
 };
 
 
diff --git a/src/Pstream/dummy/UIPstreamRead.C b/src/Pstream/dummy/UIPstreamRead.C
index c8b7286fdf..7c82adb7f3 100644
--- a/src/Pstream/dummy/UIPstreamRead.C
+++ b/src/Pstream/dummy/UIPstreamRead.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
-    Copyright (C) 2021-2024 OpenCFD Ltd.
+    Copyright (C) 2021-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -45,7 +45,7 @@ std::streamsize Foam::UIPstream::read
     char* buf,
     const std::streamsize bufSize,
     const int tag,
-    const label communicator,
+    const int communicator,
     UPstream::Request* req
 )
 {
diff --git a/src/Pstream/dummy/UOPstreamWrite.C b/src/Pstream/dummy/UOPstreamWrite.C
index 24078ca924..dfd4be78af 100644
--- a/src/Pstream/dummy/UOPstreamWrite.C
+++ b/src/Pstream/dummy/UOPstreamWrite.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
-    Copyright (C) 2022-2023 OpenCFD Ltd.
+    Copyright (C) 2022-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -46,7 +46,7 @@ bool Foam::UOPstream::write
     const char* buf,
     const std::streamsize bufSize,
     const int tag,
-    const label communicator,
+    const int communicator,
     UPstream::Request* req,
     const UPstream::sendModes sendMode
 )
diff --git a/src/Pstream/dummy/UPstreamCommunicator.C b/src/Pstream/dummy/UPstreamCommunicator.C
index e7c713f85c..29055a6da2 100644
--- a/src/Pstream/dummy/UPstreamCommunicator.C
+++ b/src/Pstream/dummy/UPstreamCommunicator.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2024 OpenCFD Ltd.
+    Copyright (C) 2024-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -38,7 +38,7 @@ Foam::UPstream::Communicator::Communicator() noexcept
 // * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
 
 Foam::UPstream::Communicator
-Foam::UPstream::Communicator::lookup(const label comm)
+Foam::UPstream::Communicator::lookup(const int comm)
 {
     return UPstream::Communicator(nullptr);
 }
@@ -56,4 +56,10 @@ void Foam::UPstream::Communicator::reset() noexcept
 {}
 
 
+int Foam::UPstream::Communicator::size() const
+{
+    return 0;
+}
+
+
 // ************************************************************************* //
diff --git a/src/Pstream/dummy/UPstreamWindow.C b/src/Pstream/dummy/UPstreamWindow.C
index 4c8e5b7906..9af10b6e91 100644
--- a/src/Pstream/dummy/UPstreamWindow.C
+++ b/src/Pstream/dummy/UPstreamWindow.C
@@ -47,4 +47,10 @@ void Foam::UPstream::Window::reset() noexcept
 {}
 
 
+int Foam::UPstream::Window::size() const
+{
+    return 0;
+}
+
+
 // ************************************************************************* //
diff --git a/src/Pstream/mpi/UIPstreamRead.C b/src/Pstream/mpi/UIPstreamRead.C
index 4f94257e71..4f5fe7e2a7 100644
--- a/src/Pstream/mpi/UIPstreamRead.C
+++ b/src/Pstream/mpi/UIPstreamRead.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019-2024 OpenCFD Ltd.
+    Copyright (C) 2019-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -47,7 +47,7 @@ static std::streamsize UPstream_mpi_receive
     const std::streamsize bufSize,
     const int fromProcNo,
     const int tag,
-    const Foam::label communicator,
+    const int communicator,
     Foam::UPstream::Request* req
 )
 {
@@ -344,7 +344,7 @@ std::streamsize Foam::UIPstream::read
     char* buf,
     const std::streamsize bufSize,
     const int tag,
-    const label communicator,
+    const int communicator,
     UPstream::Request* req
 )
 {
diff --git a/src/Pstream/mpi/UOPstreamWrite.C b/src/Pstream/mpi/UOPstreamWrite.C
index d2e1da2ec6..8ebb237546 100644
--- a/src/Pstream/mpi/UOPstreamWrite.C
+++ b/src/Pstream/mpi/UOPstreamWrite.C
@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2017 OpenFOAM Foundation
-    Copyright (C) 2019-2023 OpenCFD Ltd.
+    Copyright (C) 2019-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -55,7 +55,7 @@ bool Foam::UOPstream::write
     const char* buf,
     const std::streamsize bufSize,
     const int tag,
-    const label communicator,
+    const int communicator,
     UPstream::Request* req,
     const UPstream::sendModes sendMode
 )
diff --git a/src/Pstream/mpi/UPstreamCommunicator.C b/src/Pstream/mpi/UPstreamCommunicator.C
index 9040f2f0dc..adff0a7ea3 100644
--- a/src/Pstream/mpi/UPstreamCommunicator.C
+++ b/src/Pstream/mpi/UPstreamCommunicator.C
@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2024 OpenCFD Ltd.
+    Copyright (C) 2024-2025 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -39,7 +39,7 @@ Foam::UPstream::Communicator::Communicator() noexcept
 // * * * * * * * * * * * * * Static Member Functions * * * * * * * * * * * * //
 
 Foam::UPstream::Communicator
-Foam::UPstream::Communicator::lookup(const label comm)
+Foam::UPstream::Communicator::lookup(const int comm)
 {
     if (comm < 0 || comm >= PstreamGlobals::MPICommunicators_.size())
     {
@@ -70,4 +70,27 @@ void Foam::UPstream::Communicator::reset() noexcept
 }
 
 
+int Foam::UPstream::Communicator::size() const
+{
+    int val = 0;
+
+    MPI_Comm comm = PstreamUtils::Cast::to_mpi(*this);
+
+    if (MPI_COMM_SELF == comm)
+    {
+        return 1;
+    }
+    else if
+    (
+        (MPI_COMM_NULL == comm)
+     || (MPI_SUCCESS != MPI_Comm_size(comm, &val))
+    )
+    {
+        val = 0;
+    }
+
+    return val;
+}
+
+
 // ************************************************************************* //
diff --git a/src/Pstream/mpi/UPstreamWindow.C b/src/Pstream/mpi/UPstreamWindow.C
index 8bcce43014..0657c86096 100644
--- a/src/Pstream/mpi/UPstreamWindow.C
+++ b/src/Pstream/mpi/UPstreamWindow.C
@@ -50,4 +50,29 @@ void Foam::UPstream::Window::reset() noexcept
 }
 
 
+int Foam::UPstream::Window::size() const
+{
+    int val = 0;
+
+    MPI_Win win = PstreamUtils::Cast::to_mpi(*this);
+    MPI_Group group;
+
+    // Get num of ranks from the group information
+    if
+    (
+        (MPI_WIN_NULL != win)
+     && (MPI_SUCCESS == MPI_Win_get_group(win, &group))
+    )
+    {
+        if (MPI_SUCCESS != MPI_Group_size(group, &val))
+        {
+            val = 0;
+        }
+        MPI_Group_free(&group);
+    }
+
+    return val;
+}
+
+
 // ************************************************************************* //