Updated May 2013
itemid (Num, in, optional) allows you to specify a unique ID (in the range of 1000-32767) to assign to the item being added, or to identify the item being modified or referenced. Items can also be identified by their ordinal position or text, but these are less satisfactory because either may change at runtime due to forces outside the developer’s awareness at the time of writing the code. The use of itemid is also the only way to create nested submenus (i.e. assign an itemid to the parent of the submenu, then reference it as the Menuid of the child items).
History
2013 May, A-Shell 6.1.1351: ItemId no longer needs to be globally unique; it only needs to be unique relative to the DlgId—i.e., to the window to which the menu is attached. This makes it easier to generate menu item IDs when working with multiple dialogs, without having to worry about collisions.
