On his blog (here and here), JP Koning discusses currency status:

… laws that … grant … currency status. … Say that person A is carrying some sort of financial instrument in their pocket and it is stolen. The thief uses it to buy something from person B, who accepts it without knowing it to be stolen property. If the financial instrument has not been granted currency status by the law, then person B will be liable to give it back to person A. If, however, the instrument is currency, then even if the police are able to locate the stolen instrument in person B’s possession, person B does not have to give up the stolen [instrument] to person A. We call these special instruments negotiable instruments.