Did you find an answer?
Yes, I found the answer on stackexchange: https://apple.stackexchange.com/questions/252056/what-is-bank-account-unn
UNN stands for Unique Payer Number (ru: УНП). I don't know why it's abbriviated as UNN, may be it's just transliteration for ru. УНН, which was used in Belarus before, but now is replaced with УНП. The meaning is pretty same.
UNN is required for taxes in Belarus, Russia and may be in other countries in this region.
Sorry for late response. This forum UI is freaky and I was not able to find posts I'm following.