The Hybrid Porsche and the factory Bimmers are in E1 XP, which is basically a ru-what-you-brung class for whichever cars the organizers deem desirable, there are further subdivisions in this class, but I don't know if those have any special consequences. The Scirrocos are in AT, which is for (A)lternative Fuel cars with (T)urbocharged engines.
As for the other classes, you might wanna take a look at one of the entry lists on the
VLN-homepage as the cars there are sorted according to classes and there usually is a short description of the class I.e. SP3 means 24hour Specials between 1750cc and 2000cc, SP3T would be similar but already starting at 1600cc and with a tubo-charger. So the the letters indicate the division the cars is in (SP= 24h Specials, V= Production cars, H = Gruppe H, a German historic class that allows for a lot of technical freedom, but that is only allowed for VLN, but not for the 24hours), whereas the numbers indicate the engine displacement.