EVENT_SETDEFAULTS is Game Event.
This event is used when the game is initializing. It's the ideal place to change the value of userdefs if its only required to change them once.
It takes place after the game set the default settings, but before the .cfg files save / load them, meaning you can change the defaults settings of the game (as the name suggests). For example, if you change the value of screen_size, that will be the the status bar size that will be displayed in a clean installation.