The OnUserDefined event is fired when a script specifically triggers this event by using the SignalEvent() function.
The standard creature event handlers support throwing an OnUserDefined event for each of the other creature events. This both allows a creature's events to be implemented in a single script and allows those implementations to not displace the standard event handlers. However, the OnPerception event only throws its user-defined analog for "seen" events, and the OnHeartbeat event does not throw its user-defined analog if there are no player characters in the area. (More precisely, the OnHeartbeat event does not throw its user-defined analog if the creature in question is running at the "very low" AI level, which is the default when there are no player characters in the area.)
Objects that define this event[]