if you have a turbo/supercharger/swap/internal engine work you could also be in SM2(or if your region doesnt have CSM your stuck with SM2).
If you have a 2nd gen or an LSD(torsen or clutch only) your in CSP. Suspension is pretty much allowed as long as your not cutting into subframes.
If you have a 1st gen, with just shocks, springs(coilovers allowed), swaybars, nonmetal bushings, intake, exaust, no torsen or clutch LSD(vlsd allowed) and tires no bigger than 225s(street tires only) your in STS2.
If your car has only exaust, drop in filter, front swaybar, shocks(no spring changes), wheels no wider than stock, your in stock class.
Mind you these are general guidelines, and a lot of things are missing.