same as STX only turbo displacement is upped to 3.1, and N/A displacement is unlimited. Also, maximum front tire width 275, (awd 245).
The class accepts SRT-4s, STi's, Lancers, S4s, Camaro/firebird, etc.
Theres a little info in the RMD link in my sig.
Basically, its just STS for faster/fancier cars.