I have a kingdom of fighters, mages, halflings, and elves. My fighters and mages attain max rank without a problem because of their training buildings, but the other two races' buildings don't take them that high. When I looked at the unit screen of an elf and a hobbit at the highest level their buildings get them, they had letters after the level number. What do those indicate?

from what i have been able to dig up
the letters represent the increase in fighting ability after non humans reach max level
progressing through the alphabet
essentially the further down the alphabet the higher the combat skill
(sort of like a level but without additional HP, spells, etc)