Args
ARG(1) is whatever the "WPARAM" arg is for that event. What it's purpose is depends upon what event has happened.
ARG(2) is whatever the "LPARAM" arg is for that event. What it's purpose is depends upon what event has happened.
ARG(3) is the "message number" for the event.
Return
What you return depends upon the event. Some events may require you to return certain things.
Notes
Some add-ons, such as RexxSock, RxMidi, and RxTwain may cause events that REXX GUI does not support in its standard set of event handlers. For such events, your EXTRA event handler will be called. You'll need to consult the other documentation for details about args and returns.