I've seen a few posts that use these abbreviations, which seem to refer to Dragon Hunter and Fire Mage, respectively (and also "SLD" for Mercenary?). What do these abbreviations stand for and where did they come from?

Thanks!

They are simply abbreviations of the original German terms:

DJG = "Drachenjäger" (German for: 'Dragon Hunter')
SLD = "Söldner" (German for: 'Mercenary')
KdF = "Kloster der Feuermagier" (German for: 'Monastery of the Fire Mages')

Actually Kreis des Feuers, but it's more-or-less accurate. These are, like Philobyte said, abbreviations from German, which are also often used as prefixes for armor and NPC codes. Abbreviations for G2 from one of my earlier posts: G1 also had the following:

I'm sorry I got the "KDF" thing wrong.
It was the only item where I had to make a guess - based on the fact that all the monastery mages seemed to have it as their prefix -, and I originally intended to post my assumption with a question mark.
But then I told myself: "Come on, what else but the Monastery could it refer to?"

I'm simply not yet versed enough in Gothic terminology, it seems, to even remember the fact that there are 'Circles' of magic. Thank you for correcting me. Actually, "Bürger" does not really work here as a resolution to VLK, does it?
It would make more sense like this:

VLK (Volk) - Townsfolk

Me in the original thread: :)

But that's because I took the original group names from Mondgesaenge. I added those I knew the original abbreviations for (like Kreis des Feuers/Wassers / Bauer), but I wasn't completely sure about VLK, so I didn't want to just write "Volk-something".