the idea is to have a statistically solid conclusion if tag does or does not have the NACK bug.
-in short, ref https://github.com/iceman1001/proxmark3/issues/141
NACK bug; when a tag responds with a NACK to a 8 byte nonce exchange during authentication when the bytes are wrong but the parity bits are correct.
This is a strong oracle which is used in the darkside attack.