How does the universe naming thing work? There's the first troll universe with meenah and the gang, then the second one with vriska and friends, then the john and pals, then the roxy and crew, and now this last one. Why is it universe c? Why not e?

 Pre-scratch and Post-scratch take place in the same universe just different parts of it. The Trolls were Universe A.  Meenah’s group was A1, Karkat’s was A2. The Humans were Universe B. John’s group is B1 and Roxy’s is B2. The New Universe is therefore C.