Given a menu, items are appended which corresponds to the standard
edit commands which can be applied to a buffer with
wxMediaBuffer::DoEdit (section 
).
The idOffset parameter lets you offset the ids assigned to the menu items (so that they don't collide with menu item ids your application needs to use). If idOffset is zero, then you can use the menu id selected and pass it directly to DoEdit; otherwise, you'll need to subtract off your idOffset before calling DoEdit.
The return value is one greater than the last menu id used by the appended items. (Use this to set the offset for more menu items.)