Each communicator contains a group of valid participants; this group always includes the local process. The source and destination of a message is identified by process rank within that group.
For collective communication, the intra-communicator specifies the set of processes that participate in the collective operation (and their order, when significant). Thus, the communicator restricts the ``spatial'' scope of communication, and provides machine-independent process addressing through ranks.
Intra-communicators are represented by opaque intra-communicator objects, and hence cannot be directly transferred from one process to another.
MPI-Standard for MARMOT