When a message is sent to a procedure, a program, or a system user, it is placed on a message queue associated with that procedure, program, or user. The procedure, program, or user sees the message by receiving it from the queue.
i5/OS™ provides message queues for:
Additional message queues can be created to meet any special application requirements. Messages sent to message queues are kept, so the receiver of the message does not need to process the message immediately.