The key concept of the collective functions is to have a ``group'' communicator, and collectivegroup, for collective collective, and communicatorcollective, process group of participating processes. The routines do not have a group identifier as an explicit argument. Instead, there is a communicator argument. For the purposes of this chapter, a communicator can be thought of as a group identifier linked with a communication domain. An intercommunicator, that is, a communicator that spans two groups, is not allowed as an argument to a collective function. intercommunicator, and collective collective, and intercommunicator