Whenever a menu item is selected, HyperCard sends a doMenu message. You can respond to a menu selection by writing a doMenu handler.
If a message is not intercepted by a handler, HyperCard checks the menu item’s menuMessage, which you add with HyperTalk. If the menuMessage is not empty, HyperCard sends that message to the current card. The menuMessage can be a custom message with a custom handler or a HyperTalk command. For standard menu items, HyperCard will perform its standard action, unless you have written a doMenu handler or a menuMessage for that standard item.