Classes combine drivetrain (front-, rear-, or all-wheel drive) with three categories: Stock, Prepared, and Modified. A stock front-wheel-drive car runs as SF; a modified all-wheel-drive car runs as MA, and so on. Stock keeps cars close to showroom condition on DOT tires; Prepared allows stage-rally tires plus light suspension and exhaust work; Modified opens the door to engine swaps and serious weight reduction.