Purely arbitrary choices; I think that was the order they were in in the keycode doc I used. Nothing to stop anyone changing them to meet their needs (or indeed, not calling the translate routine at all and sorting out the raw codes if that's necessary). I didn't want the host computer to have to do that but didn't want to restrict it to the first 128 ascii codes, while leaving the possibility of using the editing functions that most people know (ctrl-c, v, x, z etc). IIRC there's space in there to fit shift-ctrl-alt-alpha as well, but I don't like using that many modifier keys.

There's an implicit ranking of shift/alt/ctrl tab, too...

Neil