State and Experimental Aviation Point Location Index

State and/or Experimental Aviation Point Location Index is a four-letter unique identifier assigned to the location of Russia's state aviation points, as well as experimental aviation points.
These indices are used to transmit aeronautical information about state airfields, airspace use requests, designation of military and sports airfields on air navigation maps. Indices are assigned by the Ministry of Defense and are not ICAO airport codes, although they use a similar format. If a state airfield has an ICAO index, a joint use by civil aviation is permitted.
Indices of state airfields are intended only for internal use within Russia. They have an external resemblance to ICAO codes for the location of Russian airports, but unlike them, they begin with the letter "Ь" when using Cyrillics or with the letter "X" when using Latin alphabet. Indices cannot be used outside Russia via AFTN.