First, I am going to move this to the Autocross/Motorsports forum. There is a thread there about classing (SS or ASP).
Classes are divided into allowed preparation levels.
STOCK This is covers most factory stock cars and mods are limited to struts, front swaybar, tires, same dia/width rims, exhaust. Some misc. They are listed in classes Super Stock (SS), and then A through H. H are the lowest potential.
STREET PREPARED Stock cars that are allowed certain modifications such as increased intake, exhaust, suspension allowances. Diff wheels. Classes are listed as ASP through FSP, wtih ASP the fastest.
PREPARED Tube frame and heavily modified cars.
MOD Purpose built race cars such as open wheel and winged beasties.
Autocross is different in various regions. In some cases, it's a tighter course that will favor the smaller car. In other regions, horsepower will make a bigger difference. For example, here in San Diego, our courses are often over a mile long and in 3rd gear. The Z06 Corvettes do very well.
It's a great way to learn (safely) car control and vehicle dynamics.
The reason why I ask is that we just sold our SMII prepared 3rd gen RX7 to a young man named Kerrick from Salt Lake City. He flew here from SLC and drove it back.
Its a Pettit motor, pettit turbo's, pettit unlimited ECU, coilover/koni with custom revalving etc etc . Very nice car. I was just wondering if that could have been Kerrick and our old car. We just sold it to him a month ago.