I believe these are arbitrary and meaningless, I mean what if RBTC has 18 decimals? Who can stop that? Yes when you withdraw to the mainchain you can't withdraw any more granular than 8 decimals, but that doesn't make 18 decimals invalid, I mean if withdrawal is the only metric then sats are invalid too because you can't withdraw dust either. The point of using micro and nano is also that it makes the weird argument about what happens if BTC is worth 100 trillion dollars irrelevant... Just use the next metric prefix in line.