The script attached to this event fires when an item is activated that it is in the PC's inventory. This is useful when a particular item must be used at a location to proceed (such as opening a door) or causes a creature to be summoned.
Whenever an item that has the item property spell "Unique Power" (targeted) or "Unique Power - Self Only" (self) casts its spell.
GetItemActivator() returns the owner of the item that was activated.
For all purposes, the module owns the event, not the PC using the item. This means, for instance, that if you make a unique power item that damages creatures, the module and not the PC will be accredited with damaging the creatures. And if the item is used to kill a creature, the PC will gain no XP for this, as the module will get credit for the kill.