It's a Russian internal city or airport identifier. Many small cities don't have IATA codes, but do have internal Russian codes.
This system is called CRT (Center for schedules and tariffs, ЦРТ, Центр Расписаний и Тарифов) named after a department in the organization that manages them (see below). Wikipedia article cited below claims it's also called "Sirena" code (named after Sirena -- a booking system for airline and railway tickets).
Here's a translation of a fragment from Russian Wikipedia:
The internal city and airport coding system in the former USSR is structured analogous to IATA. [...]
The airport code in this system consists of 3 letters of the Russian alphabet. As in IATA codes, the 3-letter combination is interpreted as atomic and has no internal structure. As in case with IATA, all 3-letter codes are unique.
Like IATA, the codes were assigned to cities such that the code has something in common either with the name of the city or with its IATA code [...] Contrary to IATA, the internal codes were changed after cities were renamed in the 1990s: Saint Petersburg's internal code was changed from ЛЕД to СПТ, Samara (formerly Kuibyshev) -- from КУФ to СМШ, Ekaterinburg (formerly Sverdlovsk) -- from СВХ to ЕКБ.
Many small cities and airports in the former USSR have an internal code only.
In late 1990s foreign (non-Russian) cities were assigned internal codes because of the popularity of these destinations (e. g. Barcelona -- БЦН).
As of April 2009, there are about three thousand internal codes that are actively used.
It appears that the codes are maintained by the Transport Clearing House. The section Coding Directories on their page Schedule & Tariffs says:
STC maintains the aggregated database of coding directories for the purpose of universal use of codes in information systems for passenger services reservation and sales, publication of timetables and fares, ticketing, settlement and other commercial issues related to passenger services.
STC maintains the codifiers in pursuance of Provision on Assignment, Maintenance and Use of Internal Codes of Carriers, Organizations, Airports, Cities (points) and Civil Aviation Agencies (№ ДВ-63/и dated 12.05.94) and Directive "On Coding Directories Approval” (№ ДВ-109/и date 14.07.94).
STC supports the following coding directories:
In Russian, STC is translated as CRT (ЦРТ) and I've heard people refer to the internal codes as "CRT codes".