They are autonomous. They aren't federal and there is no reserve. Moreover the money they spew isn't to be found in the Constitution either as CONgress was only given authority to "coin" money and regulate the value of foreign coin. The Coinage Act of 1792 specifies in no uncertain terms what a dollar is—The Act established or
defined the "dollar" as a weight of silver (371.25 grains (troy) of fine silver) and then regulated the value of gold coins to it in a 15 to1 ratio, that is, as 15 grains of silver to every grain of gold. The regulation also included establishing the purity and the various denominations.
Thus the Coinage Act gave the new nation three gold coins (the Eagle or $10 gold piece, Half Eagle and Quarter Eagle); five silver coins (the Dollar, Half Dollar, Quarter, Dime (originally spelled Disme), Nickel (or half Disme); and two copper coins (the Cent and Half Cent). This is Constitutional money. Moreover, the Act provided all citizens access at the Mint to coin their gold, silver, and copper (free coinage) and established any debasement of the coinage as a capital offense!