Most of the car in Lego City have a sticker license plate. The number on a license plate is corresponding to the Lego Set number. But what about these extra letter there? Is it just to add more realism? Are the initials of the particular designer?
The letters typically represent the initials of the set designer. See more discussion on this topic in this EuroBricks forum thread.
Some other sets also carry set designer initials, sometimes subtly incorporated in the build (I believe Jamie Berard, the famous Modular Houses designer, puts his initials in some of the builds), other times on the lettering on the sticker sheet. It is definitely not standard practice.