Complex sentence: Usually these will be threads all belonging to the same process, but they may also be from different processes, for example when the processes have a producer-consumer relationship, or when they all come from the same MPI program.
Split into simpler sentences: Usually these will be threads all belonging to the same process, but they may also be from different processes. For example when the processes have a producer-consumer relationship, or when they all come from the same MPI program.