A unique item identifier (UII) is not the same as a unique identification number (UIN). When the UII is the serial number of an item such as a vehicle identification number (VIN) on a car, then the UIN is the license plate on the back. When the UII and the UIN are linked on a database, a uniform identification system (UIS) is created.

The UIN must be one-of-a-kind

If a VIN is the equivalent of a UII, it stands to reason that the license plate number must be unique as well. The strategy doesn’t work if duplicate plates are possible. If everybody from Iowa could just make up their own license plate numbers, many of us might choose 000 001.
https://en.wikipedia.org/wiki/Vehicle_registration_plate

Linking the UII and UIN on a database

When we buy a car, the process of linking the VIN to the license plate is fairly seamless because the strategy was first implemented at the turn of the 20th century and has been continuously improved ever since. So ubiquitous is the process, that most people don’t understand that it’s a critical tool for law enforcement professionals.
https://en.wikipedia.org/wiki/Motor_vehicle_registration