An X reg car is a vehicle registered in 2000. A single year identification letter is part of the prefix registration format in the United Kingdom from 1983 to 2001.
Prior to 1983, single letters were used as suffixes in registration plates. These single letters were called "age identifiers." The prefix registration format was divided into three parts - an age identifier, a number that contains one, two or three digits, and another three letters that usually indicate the location where the registration was released. Age identifiers were only used for vehicles that matched the year of registration or those more recent than the year the registration was issued.