I am not familiar with the letters but if you bought a First Class/Buz Class ticket (I believe they dont sell FC anyways) you will be seated in the First Class Section of the aircraft. If you bought a regular economy ticket, as RayBolt explained, you might end up in Buz class.
I ended up on Buz class on my way to AUA
in the DFW
sector. We checked in really early in the mroning for the COS
flight and we were dispersed through the aircraft and my dad asked if we could be placed together and they placed us all in Buz .
So if I were you go early and ask if they have any seats available between rows 9-13.