Intra-communicators bring together the concepts of group and context. To support implementationspecific optimizations, and application topologies (defined in the next chapter, Chapter Virtual Topologies for MPI Processes), communicators may also ``cache'' additional information (see Section Caching). MPI communication operations reference communicators to determine the scope and the ``communication universe'' in which a point-to-point or collective operation is to operate.
Each communicator contains a group of valid participants; this group always includes the local MPI process. The source and destination of a message are identified by MPI process ranks within that group.
For collective communication, the intra-communicator specifies the set of MPI 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 MPI process addressing through ranks.
Intra-communicators are represented by opaque intra-communicator objects, and hence cannot be directly transferred from one MPI process to another.