There is a problem with the numbering ID's of having extra sub categories which go beyond Z in the alphabet.
So for example if the parent category ID is 15 - then the subs follow on at 15a, 15b, 15c etc etc. Once Z is out of the way it goes 15aa and then logically should go 15ab, but it doesn't - every subcat thereafter remains at 15aa.
This would probably cause future complications for exporting and importing listings.
See pic below - this is what I have on one of my subcat lists and just replicated and spotted on another: