C IS FOR SOFIA, A STANDS FOR BURGAS
Number plates usually reflect the year of the first registration, or the province where the car's owner resides, or sometimes they give out nothing at all except a unique combination of letters and numbers detectable by the traffic authorities and the police.
Theoretically, they should give out meaningful information in Bulgaria as well. But try to find out what an Y stand for on a local number plate and then think of the TX on another, and you are bound to see that not even number plates in this country are produced the way things are done elsewhere.