game_action_causer is a marker interface. I haven’t worked with it myself but it looks like the only class currently implementing the interface is fort_character. I suspect that in the future as the APIs are expanded other classes like weapon and vehicle will also implement this interface.
To work with damaged_result you would attempt to cast the game_action_causer to different types to determine what type of object caused damage to the player. For example:
OnDamaged(DamagedEvent:damaged_event):void =
if (CauserCharacter := fort_character[DamagedEvent.Source?]):
# Game code...