system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] In the Republic, clone troopers had serial numbers starting with CT (Clone Trooper) or CC (Clone Commander). The Empire and First Order used a similar number system, using the prefixes TK and FN, respectively. Did these stand for anything?

Yeah, I know, kind of a minor detail. But overthinking minor details is half the fun of this subreddit. 

Just curious if there are any answers, official or otherwise.
gpt
Right now nothing is conclusive, but the gist is that they're "Legion Numbers" in base 26. For example the 501st would be the SG legion because S would be the 494th plus G, which is the seventh. Finn was confirmed to be in the FN legion and iirc the TK legion was on the Death Star.

So they're not abbreviations, they're just numbers.