The new optional maxCommsSize in the etc/controlDict sets the maximum size in bytes to used for unstructured data exchanges. In some phases of OpenFOAM the code can send over very large data chunks, e.g. during parallel load balancing, causing problems for some Pstream implementations.
Setting this variable to greater than zero indicates that the data exchange should be performed in multiple passes, using a maximum message size of maxCommsSize bytes.
This is not switched on by default since it requires an additional global reduction, even if multiple passes are not needed.