Thank you!
They are extra cards and can be used like gaff cards. I think they meet most of the definition a gaff card:
– Same card back like the whole deck
– Extra cards can be used within the deck
– Magicians can use them to perform tricks
– Spectators would be unaware of any unique cards in the deck
This may not fit the perfect definition of a gaff card, but I like to refer to them as gaff cards. I suppose it also depends on how creative the magician is 🙂