It has become traditional (ie. online slang that has been in use for at least a year) that when you’re running a raid and have several tanks in the group, one is referred to as the Main Tank (MT) and the others are Off Tanks (OT).
It’s role related. The MT picks up the biggest baddest monster and the OTs have other jobs which might involve grabbing adds, building threat on the main target (but staying below the MT), taking over if anything happens to the MT, and so on.
But there is a cachet involved with being the main tank which is kind of weird. I mean, all of those roles are equally important and it’s not as if being MT takes any special skill.
Last week, we were chatting on our sekrit tanking channel during the raids and a lot of it was deciding before each fight who was going to tank what. There really wasn’t one main tank, we changed it around for each fight.
So I think I’m going to start referring to the rest of the tank corps as Co-Tanks. I think Off Tank/ Off Healer/ Off Spec etc is a bit derogatory.