Before all paper money became Federal Reserve Notes, some was legally required to be "United States Notes", which were direct obligations of the US Treasury (as opposed to coming from the Federal Reserve system). Before the 1970s $2s and $5s were issued as USNs, with a red seal instead of the green seal of FRNs (or the blue seal of Silver Certificates, which had a slightly different legal authorization and obligation). When the concept of redeeming paper money for silver ended in the 1960s, USNs became irrelevant, and the $2 and $5 USNs were pulled from circulation and eventually reissued as FRNs. But the treasury still had the legal requirement to circulate a certain dollar amount in USNs. So they printed up a bunch of $100 USNs, stuck them in boxes, and occasionally carted them around between different bank vaults, which technically met the definition of "circulate". This went on until 1994, when Congress finally repealed the law requiring the USN circulation.