https://en.wikipedia.org/wiki/List_of_UIC_country_codes

The UIC country code determines the country of origin of a vehicle. The owner is identified by a trailing letter code. The meaning of these digits was changed at the beginning of 2006. Previously, these two digits formed the ownership feature, with state railroads and some large private railroads in particular receiving their own codes. Other private railroads were given a group code or they placed their wagons in the park of a state railroad as private wagons. Such wagons were marked with a P in a rectangular frame behind the wagon number.