SOFREP reported on this but didn't bother to source this information in the article, so there's no attribution. But here's what they say:
https://sofrep.com/news/what-do-those-letters-mean-on-russian-tanks-and-vehicles/
The Letter “V” represents the Russian Marines
The Letter “Z” represents units in the Eastern Military District
The Letter “Z” in a square represents the Southern Military District (Crimea)
The Letter “O” represents units from Belarus
The Letter “X” represents the forces of Ramzan Kadyrov, the Putin installed warlord of Russia’s Chechnya region.
The Letter “A” represents Russian Special Forces(SSO) like Spetznaz in its various operational units.