WHEN:  to decide if you will allow the user to enter the control (RETURN .F. 
       prevents access to the control)

GOTFOCUS: To react to the user getting into the control.  GotFocus fires after 
          When has returned .T. to allow them in.

VALID: To determine if you will allow the user to leave the control. (RETURN 
       .F. or RETURN 0 prevents the user from leaving the control)

LOSTFOCUS: To react to the use leaving the control.  (LostFocus fires after 
           Valid has returned .T. or a number other than 0).