Available memory limits the maximum size and number of outstanding messages the system can handle. The number of file descriptors (I/O channels) available to a process limits the number of direct route connections a task can establish to other tasks, and the number of tasks a single pvmd can manage. The number of processes allowed to a user limits the number of tasks that can run on a single host, and so on.
An important thing to know is that you may not see a message when you reach a resource limit. PVM tries to return an error code to the offending task and continue operation, but can't recover from certain events (running out of memory is the worst).
See § for more information on how resource limits affect PVM.