Bonus Class is an extra class of cars FlatOut 2 and FlatOut: Ultimate Carnage. In FlatOut 2, you can unlock the bonus cars by using the cheat code "GIEVEPIX". InFlatOut: Ultimate Carnage you can unlock these cars in Carnage mode.
Bonus cars are the experiment with "what's gonna happen if I will make all statistics for this car only 10.0?" So, some of these cars are incredibly strong, others are ridiculously fast. Bonus class is making the game even more fun and crazy than it already is.