Intracommunicator and MPI_COMM_WORLD

http://static.msi.umn.edu/tutorial/scicomp/general/MPI/content_communicator.html

• MPI_COMM_WORLD is the initially defined universe intracommunicator for all processes to conduct various communications once MPI_INIT has been called.
• In MPI-1, where the processes of executing the MPI computation, MPI_COMM_WORLD is a communicator of all processes available for the computation. This communicator has the same value in all processes.
• In MPI-2, where processes can dynamically join an MPI execution, it may be the case that a process starts an MPI computation without having access to all other processes. In such situations, MPI_COMM_WORLD is a communicator incorporating all processes with which the joining process can immediately communicate. Therefore, MPI_COMM_WORLD may simultaneously have different values in different processes