In Nitro Nation there are many various car classes. License for them usually unlocks when you complete Career chapters. But a little aside of them all there’s one more class - F, cars of which you can see in Car Dealer if you filter his offering by F class.
F Class exists separately from other car classes and can (and should) be considered a light-hearted joke. It includes non-existing (well it depends on what you believe) cars that usually appear in the game during special holidays. For example, Santamobile - for Christmas, Love Rider - February 14th, The Ripper - an ambassador of Halloween.
This class unlocks together with B1 Street when you reach 4th Career stage in the game. These cars stats and balance are not taken into account when we balance game events and tournaments.