To happen, event usually needs to fulfil certain conditions, known as triggers as they are scripted in the trigger bracket. It's a list of one or more conditions that has to return true to allow the event to fire. As there are two kind of events, province events and country events, there is also two kind of triggers : province scope trigger will check properties of the province, while the triggers of a country event will check country properties. A same event could check both provinces and countries thanks to the scopes.
Note that all the listed triggers should work normally in events, decisions, missions, spy missions, and triggered modifiers assuming you use the correct scope. Other contexts (ideas, buildings, rebel_types, etc.) may only allow some subset to work. Quote from one of the developers regarding scopes/triggers in buildings.txt: "All normal triggers except region and continent work in buildings.txt. The things that don't work there are event targets (ally, owner, controller, capital_scope, sea_zone, emperor, revolution_target, crusade_target, this, league_leader, overlord, from, local_enemy, heretic). This is because those event targets need to look up provinces when they are initialized, and buildings are initialized before the map. Because of dependencies between different parts of the game, the initialization of files is a real tricky puzzle of catch 22 so this is not something that will be changed at the moment."
The two links above document the quasi-exhaustive list of triggers :
- Full list of country scope triggers conditions
- Full list of province scope triggers conditions
- Full list of double-scopes triggers conditions
For more info on writing/modding events, please see structure of events.
