I've read that they are force identifiers. They are not Cyrillic alphabet letters, so it makes them unique identifiers (except for O). Z may stand for "zapad" (west) and V may stand for "vostok" (east). I'm not sure about O, but O is close in shape to the Cyrillic letter for "yu" which would go with "yug" (south). Zapad and Vostok have been used as military exercise designators.
I've read that they are force identifiers. They are not Cyrillic alphabet letters, so it makes them unique identifiers (except for O). Z may stand for "zapad" (west) and V may stand for "vostok" (east). I'm not sure about O, but O is close in shape to the Cyrillic letter for "yu" which would go with "yug" (south). Zapad and Vostok have been used as military exercise designators.