Edit: This applies only to INI files which are used for ARE spawns.
I found the current description of this resource a bit confusing so after observing in-game and observed script references to these files in IWDEE I wanted to add some clarification.
Under [spawn_main], 'enter' and 'event' are different in that 'enter' is triggered each time the player enters the map, while 'event' may happen instead of or in addition to that. Both can be controlled by variables, both use the interval to determine a delay before/between spawning critters (with 1 meaning spawn now if possible) and both