Hey sorry but in a couple questions I saw people referring to sessions as things like B2 and such what does that mean?

It’s universes!
The letter dictates the universe and the number the session.
The trolls universe is the first one that created the humans’ so it’s “A”. Meenah’s session was the first in that universe so it’s A1, Karkat’s was second so it’s A2.
Human’s universe is second so it’s “B”. Beta kids’ session was first so it’s B1, Alpha’s session is second so B2.
Caliborn’s session is C1 due to being the first session in a third universe.

It’s just shorthand to make specifying what universe we’re talking about. It helps when the topic is a universal constant that happens in all sessions like Prospit, Jack Noir, and Detse.