eCash Tokens and fm-BTC
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Throughout the guide we refer to eCash tokens, fm-BTC (Fedimint BTC) or fm-sats (Fedimint satoshis) as the currency used within the Fedimint.
These terms have been chosen to represent the fact that the circulating currency in the Fedimint is not on-chain bitcoin1, but a claim on bitcoin that is controlled by the federation.
In reality the eCash tokens are claims against the bitcoin that is held in a multi-signature vault by the federation guardians. See What is a Fedimint.
The fm-BTC therefore is a means of the federation keeping track of the outstanding claims for the bitcoin which is held.
These claims are not cryptographically enforced, but instead rely on honest federation guardians to honor the withdrawals.
1 On-chain referring to bitcoin that is unencumbered on the bitcoin time chain with the private keys controlled by the user.