PythonExpireCheck- thats the Python function which is called to see whether the event has expired. It will be called every turn for every event that has occurred. If the event has expired, it gets reset. The practical use of that is for quests. Events that set a quest should sent a Python expiration check function, which will determine the failing conditions of that quest.
Using Python expiration functions will not let you undo regular (non-quest) events. So if you have an event that, say, adds a promotion to all melee units, and add an expiration check to that event, it wont remove the promotion when the expiration function returns true.
Expiration functions should return a true value for when the event has expired and a false value for when it hasnt expired.
Its important to note that expiration functions are only required if the expiration condition can not be described by XML alone. If the only expiration condition you want is, for example, a technology, then you can make the event expire by setting <ObsoleteTechs> in Civ4EventTriggerInfos.xml. Thats the general idea of event Python functions they complement the XML when it cant describe what you want.
