Question about MAME ROMs

In Emulators / NES ROMs

Can somebody please tell me the difference between the ROM versions of a MAME game? Let's take "Street Fighter 2010 II - The World Warrior" as an example. I know that sf2 is the master ROM and that there are versions for different countries. But let's say I want to play the American version. What's the difference between sf2ua, sf2ub, sf2ud, sf2ue, sf2uf, sf2ui and sf2uk? Is it like the PRG0 and PRG1 from NES ROMs, so that A was the first version while B, D and the others are newer revisions or how's it working? And why don't they name it A, B, C, D, E, F, G, but A, B, D, E, F, I, K?

Here are a couple very informative sites on MAME.

This one is a searchable MAME game database (Very useful if you are trying to find what NES ROM sets are for what parent NES ROM).