More of a two part question. In the current system (Fiat only currency), why is a Federal Reserve Note booked as a liability (on the Fed balance sheet), but a Coin is not booked as a liability by the Treasury? What is the difference? What liability does the Fed have since it doesn't actually have to redeem the notes for anything (other than another note?)
Also, what does it mean to say that the notes are 'obligations' of the US Government?