From Xojo Documentation
Fires when a contextual menuitem HitItem was selected but the Action event and the MenuHandler for the menuitem did not handle the menu selection.
This event gives you a chance to handle the menu selection by inspecting the menuitem’s Text or Tag properties to see which item was selected. Use this in conjunction with ConstructContextualMenu if you have not specified the Action event or the Menu Handler for the items on the contextual menu.
The following code in a ConstructContextualMenu event builds a simple contextual menu. The parameter Base as MenuItem is passed in as a parameter.
Return True // display the contextual menu