[1] They were specified in the Federal Reserve Act of 1913 and had the same value as other kinds of notes of the same denomination.
Additional denominations of $1, $2, and $50 were issued in 1918 as an emergency replacement for Silver Certificates, which were temporarily removed from circulation under the Pittman Act.
The wording, "Or by like deposit of other securities" was added after the phrase, "Secured by United States bonds deposited with the Treasurer of the United States of America".
[4] This emergency issue of notes was prompted by the public hoarding of cash due to many bank failures happening at the time.
But while they look very similar, and both have the words, "National Currency" across the top of the obverse, they had different issuers and are considered to be distinctly different types of bills.