This read-only variable returns the number of the event currently being called, where the number is actually referring to the "sub event" of the event, ie: for the step event the event number could be any one of the constants ev_step_normal, ev_step_begin, or ev_step_end. For a full list of constants that are available for the specific sub-events see event_perform, and if you should need to know the main event itself, you should be checking the event_type.
case ev_step_normal: show_debug_message("Step event!"); break;
case ev_game_start: show_debug_message("Game Start""); break;
case ev_room_start: show_debug_message("Room Start!"); break;
The above code could be called form a script and used to show debug messages informing you which event is being currently triggered.