The property for which the value is returned by this macro is the sequence number of the event, which is equal to ^$Event("SEQUENCE").
Typically, one doesn't worry very much about the sequence in which the events occur: the events happen when the end-user manipulates the mouse and the keyboard, and the various pieces of "action logic" are executed when the corresponding events happen.
On occasion, one might wish to record some or all of the events, and keep track of the sequence in which they occurred.
log(event,text) ;
Set ^log($j,%%EvSequence)=event_": "_text
Quit
and several pieces of "action logic" start with:
Set remind=...
Do log(%%EvType,remind)