The certificate of registration for a privat car consists of 2 parts, a blue and a yellow document. Part 1 should be in the car when you’re traveling, part 2 should be stored safely and not in the car (for example at home) because this part is used for changes of ownership or de-registration. If you make any changes on the vehicle that results in the information on the registration cerficate not being correct anymore, for example changing the engine type or dimensions of the vehicle, then it has to undergo a registration inspection within one month.