Why do some US 5 dollar bills have red seals?

Red seals on bills printed from 1928 to 1966 indicate they're United States Notes. The color distinguishes them from silver certificates (blue) and Federal Reserve Notes (green). US Notes were mostly issued as $2 and $5 bills but at one time or another nearly every denomination had at least one series of US Notes.

US Notes were issued directly by the federal government rather than through the Federal Reserve System. The two kinds of currency served the same purpose so production of US Notes was suspended in 1971 to save on printing and distribution costs.