I emailed the makers of KbdEdit for their input, and they just replied with the answer!
The zero-based position within ModNumber array defines the modifier combination: eg, the last element "2" is at position 6, whose binary representation is 110, ie KBDCTRL | KBDALT, ie AltGr (www.kbdedit.com/manual/low_level_modifiers.html#AltGr) The value "2" means that AltGr mappings will appear at index 2 in all aVkToWchX[] arrays (for X>=3).
The position 3 corresponds to Shift+Ctrl (= 011 = KBDSHIFT | KBDCTRL) - you see that this combination is to be found at aVkToWchX[4] (for X>=5)
If you open the German layout in KbdEdit, you will see that indeed AltGr is at position 2, and Shift+Ctrl at position 4 (zero based) - see attached screenshot.
Hope this helps.
Regards, Ivica
Thanks Ivica!