HyperCard sends some commands automatically in response to events, just
like system messages. But unlike system messages, if the command passes all
the way to HyperCard without being handled, HyperCard performs a built-in
action.
HyperCard sends choose and doMenu as messages when the user chooses a tool
or an item from one of HyperCard's menus. It sends close as a message when
the user clicks the close box of a window.
HyperCard sends help as a message when the user chooses Help from the Go
menu (or presses Command-?).
HyperCard sends arrowKey, commandKeyDown, controlKey, enterInField,
enterKey, functionKey, keyDown, returnInField, returnKey, and tabKey as
messages when the user presses one of those keys key.
This article is adapted from the Claris Tech Info database.