Different companies have different structures, but the basic format is that PSMs supervise SMs who supervise ASMs. (In a smaller company, either PSMs or SMs may be omitted.) The structure does not necessarily dictate who will prepare the promptbook or who will call the show, just who has administrative responsibility to supervise and coordinate the remainder of the staff: on an extremely large production, the PSM might have no major production duties at all and exist entirely to take care of scheduling, staff and resource coordination, reconciling notes and information from their subordinates, and so on.
