When the parking control officers do their daily rounds, they will check the license plates of every vehicle parked in a carpooling-reserved spot. By providing your full license plate in the app you are helping to ensure that your vehicle is rightfully parked in your reserved spot and that the parking control officers do not notify you by mistake.
You can change your license plate by clicking on Settings -> Car settings -> License plate.
Please make sure that you write down your EXACT license plate without any characters missing or hidden.
Do you regularly drive more than one vehicle? Then check out this article as well.