when you send a NODEFAULT this means that you don't want the event to do what it is supussed to do, or what you told him to do, e.g.
in the KeyPress event if you do something like this

*KeyPress event
if nkeyCode = 65 Then  && This is the code for the [A]
    NODEFAULT
endif

you are telling the event that if the pressed key wa the capital [A] just avoid it. The event is supused to write the A in the control where you are or in the buffer, but if you send a NODEAFULT then you r saying "For get about the last key, ok"

hope this makes sence