This feature was created for implementing Patch #1773587 at http://sourceforge.net/tracker/?func=detail&aid=1773587&group_id=33142&atid=407383 with some modifications as talked about in the patch..
Provide the ability for event generators to request notification from the Event service when all registered processes have acknowledged receiving the event. When generating an event, a process may give the option "NOTIFY". When all processes that should be notified of the event have acknowledged it, the Event service will queue a message to the process that generated an event. The message will have type STAF/Service/Event/AllAcksReceived and the message will contain the event ID of the event which has been acknowledged by all.
Since you can only generate an event for event types that have at least one registered process, my comment in the patch regarding that there must be at least one process registered for the generator to be notified is irrelevant.