A MSG-NET-EVENT occurs when a .NET control has fired an event. EVENT-DATA-2 contains the control's event type. When a .NET event type is referenced in your program, the name must be preceded with an "@" character.
An example of event handling code might look like:
USERCONTROL-EVENTS. EVALUATE EVENT-TYPE WHEN MSG-NET-EVENT EVALUATE EVENT-DATA-2 WHEN @UserControl1_FireCalc PERFORM display-update END-EVALUATE END-EVALUATE.