The default item is the menu-id that will be returned if the main menu option is clicked on.
┌──────────────┬────────────┐
│Open (->)│ Icon │ id=MID_ICON
└──────────────┤ *Tree │ id=MID_TREE
│ Details │ id=MID_DETAILS
└────────────┘
In the example above, where MID_TREE is currently the default, if the user clicked on the "Open" option without opening the conditional cascade menu, the menu would send back a notification that MID_TREE was selected.