If you're truly confused about lawful money then you've come to the right place. STSC is the premier site on the interwebs for learning about it. Here's a legal definition that may help:

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LAWFUL MONEY OF THE UNITED STATES: includes coin, paper or electronic money issued directly by the U.S. Treasury Department or the Postal Service, but excludes any form of money issued under the direct authority of the FEDERAL RESERVE CORPORATION. The distinction between currencies is a crucial distinction to understand for procedural reasons. The “elastic” or insured currency causes an income tax nexus that is governed by admiralty law, and generally converts BASIS PROPERTY into EXCISE INCOME, but publicly-owned LAWFUL MONEY OF THE UNITED STATES is governed by FEDERAL COMMON LAW and the possibility of jury oversight. LAWFUL MONEY OF THE UNITED STATES, whether issued by the Treasury Department or the Postal Service, is normally processed by the Federal Reserve system even though the Federal Reserve had no part in its creation. Again, when LAWFUL MONEY OF THE UNITED STATES is processed by the Federal Reserve system, its role is Constitutionally strictly limited to that of FISCAL AGENT of the Treasury Department.
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