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“Accounts” are private/public keypairs. You don’t have a username/password at a specific instance, you have a public/private keypair you can use to authenticate your identity. It’s like your name vs an e-mail address/username. An e-mail address is an account you have at a specific entity, this is like an activitypub username [email protected]. Your name isn’t tied to a specific instance or even a single government database (since you can be identified by name in databases of multiple governments), it’s just your identity. Your key in nostr works just like an identity or name in that sense.