Both are the same thing. In the present system what gives fiat money its value is that they are the only means to pay taxes (collected by force or thread of) and legal tender laws (same).
In general fiat currency is defined as a currency imposed by force, usually by governments. In the present system the specific mechanism are the two noted above.
Aside from taxes and and certain types of debts that may not be strictly voluntary, how do legal tender laws impose anything on someone who hasn't willingly incurred a debt?