As more and more car setups are used from this list, it is time to tell about more of them.
1LX and Formula XR Junior are the only known cars which are set to have restriction of using different tires. 1LX is supposed not have at all Soft tires available and Formula XR Junior is supposed to not have R3 tires in same fashion.
GT4 cars were set to work at first, where no cars could have use any amount of downforce, but because this option is still not available to check, in case someone could cheat, the class was set to work as average from all tracks, where now XF4 is the fastest in straights and FZ4 slowest. In other hand, UF4 alongside with FZ4 are the fastest cars in corners and XF4 slowest. Other two being somewhat between of mentioned cars.
The 1-Class works best on tracks which has long straights and not so many tight corners. Aston Historic is one good example of it.
Standard class, which contains XFG, XRG and UF1, is not valid anymore in command with /cars=STD. Also, /cars=FR would not work either.
GT9 and other minor classes are not widely used anymore, and might get notification of not used anymore.
GTP class is still not tested and is not valid class.
Notice as list is updated since last year, it is often checked this thread in case plausible errors occurs, most likely on new classes.