That's just a way of showing the order you attack enemies - say you fight a one monster and then two more of different types, they would have the letters proceeding in alphabetical order in order of how you attacked them(first one 'A' then the second one 'B' then the third one C' and so on, etc). It's just a way of keeping track of enemies you fight (it's something that first appeared in FFXII and has been revived for FFXIV).