6 new cars appear and 6 cars dissapear every time you:
-Start a race, even if you just restart right after you have finished it, 6 cars will rotate at the UCD
-Start a license test, the same test can be repeated as many times as you want without rotating at the UCD. Entering one, drive it once, then quit and enter the same again will probably rotate the UCD.
-Start a Special event, same rule as the license tests, won't rotate unless you select a new event.Entering one, drive it once, then quit and enter the same again will probably rotate the UCD.