Just a guess on my part but with alpha characters you have the possibility of 26 single character categories or places instead of 10 (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), but as they do in 17-digit VIN's they don't use I, O, or U because they could be mistaken for 1, 0 (zero) or V but I'm surprised they use the letter Q which is also not used on a 17-digit VIN. Anyway they have 23 possibilities for codes. Then they used the last numeric character of the option or item number. And in some places they used a sort of abbreviation of the code/item like PNT for paint code, AX for axle code, TRM interior trim,...
Then they also used upper and lower case letters for another 23 or 46 total code placements...again just a guess
MikeR
