Communicators are groups of processes in MPI. Messages sent on one communicator cannot be received on another, even between the same ranks with the same tag. MPI_COMM_WORLD is the default, including all processes launched by mpirun. Within a communicator, each process has a rank (0 to N-1) and the group has a size.

MPI_Comm_split partitions an existing communicator into sub-communicators. Each process passes a color to determine which group it joins and a key to control rank ordering within the new group. Processes with the same color end up in the same new communicator. This is useful for creating communicators that span a single node, a single row of a process grid, or any other subset.

int color = rank % 2;  // even and odd ranks form separate communicators
MPI_Comm half;
MPI_Comm_split(MPI_COMM_WORLD, color, rank, &half);
// use half...
MPI_Comm_free(&half);