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.)