From Valve Developer Community
- Fired when an entity starts touching this trigger. The touching entity must pass this trigger's filters to cause this output to fire. (activator is the toucher)
- Fired whenever the trigger is activated. (activator is the activator)
- These outputs each fire in response to the firing of the like-numbered
FireUser4Input; see User Inputs and Outputs.
OnKilled(Only in the Left 4 Dead series)
- This output fires when the entity is killed and removed from the game.