This variable is constantly used to 'return' important values, such as those in events.
onevent EVENT_GETLOADTILE setvar RETURN <value> endevent
In this case, RETURN "return"s the load screen background number into the var RETURN, setting it to something else changes it, changing the background for the loading screen.
onevent EVENT_FIRE setvar RETURN -1 endevent
In this case, setting RETURN to -1 cancels EVENT_FIRE, causing the player to no longer be able to use a weapon.
espawn LIZTROOP setactor[RETURN].pal 21 setactor[RETURN].extra 400
In this case, RETURN will allow you to spawn an Assault Captain [LIZTROOP, Pal 21] with 400 health instead of 60 health.