Banished face-down cards are not public knowledge regardless of the fact the owner knows them and is allowed to check them. They cannot be used for any effects that would require a specific card
- level/rank ect.
Cards that just involved "Banished cards" can use Face down cards such as "Necroface" and "psy-frame lord omega". Cards like "Burial from the different dimension" and Dimension fusion cannot be used as they require certain types of cards and not just "Cards".
Face-down banished cards are also able to be used for verficiation to prove you don't have the legal copies of a card in your hand/deck.
ex. If "White stone of legend" Went to your graveyard but all 3 copies of "Blue-eyes white dragon" are banished face down you may reveal those banished cards. They however remain banished face down and are still not public knowledge regardless of the fact both players know them. The game will see them as just "Cards"