It's a unique and simple registration (or serial number if military I think) rather than a tail number and serves to identify a particular airframe when on the official books of a particular country. Like car registrations.
Aircraft construction numbers (Manufacturer's Serial Number) are often not unique, eg 10345 on RZJets turns up four airframes, F-27, DC-4, A321 and a CRJ. Probably plenty of others not listed on their DB.
Incidentally, I'd forgotten about the Soviet-era CCCP - which when westernised should read SSSR-. Good job they didn't choose a Socialist nomenclature made up of some other Cyrillic letters, might have proved a challenge for plane spotters using typewriters and so on to log their sightings in the days of yore (I used small Index Cards and a typewriter a friend gave me...)
Last edited by treadigraph; 3rd December 2025 at 12:16.