Legal tender has a very specific purpose in tax law. You use legal tender as the base currency to calculate capital gains tax.

If you live in the United States the government expects you to use USD as the base currency for capital gains tax.

However, if you live in Australia and the government declared AUD as legal tender but you're holding some USD you are supposed to pay capital gains tax if you sell, dispose or transact with USD.

You could technically walk into a coffee shop in Australia and pay with USD if the owner accepted it. But that transaction could incur a capital gains tax for you and the coffee shop owner now has the USD and they could incur capital gains tax if they converted it to AUD.

In other words, legal tender is simply the currency the government declared it to be.

The point is, even though USD and AUD are both legal tender in thier respective countries they are not equally treated as legal tender from country to country.

Legal tender is any form of money that courts of law are required to recognize as satisfactory payment for any monetary debt. In particular, legal tender is typically the currency the government collects taxes in. However, the money itself does not technically have to be a debt based fiat currency. Although, as far as I'm aware they all are.

Remember, laws are just rules some politicians decided. There's nothing technically stopping them from deciding to make sea shells legal tender.