I think I am not the only one to find this matter confusing. The one clear and authoritative source of information is a pdf, in Russian language and Cyrillic alphabet only: http://www.caiga.ru/DocAni/manual_of_4_letter_indexes/Indexes_of_Airports.pdf ; it is regularly updated.

There are three columns of codes:

• local “civilian” code, usually beginning with ‘У’, which transcribes to U

• local code for “state” airfield, which includes military terrains, codes usually begin with a ‘Ь’ character, which transcribes to ‘X’

• “international” code, given in Latin alphabet, corresponds to ICAO

Aerodromes of mixed military/civilian use will have the first and second columns filled out; or, if they have international status, all three. UHSS Yuzhno-Sakhalinsk is an example.

Thanks to mapper Mazda05 for patiently explaining!

PS local_ref beginning with Z or H are not official, they seem to be empirically assigned by maps.aopa.ru (which I often consult, though its information is not always perfect). Better an unofficial ref than none at all, in my opinion.