[Documentation](https://nix.dev/manual/nix/2.28/command-ref/new-cli/nix3-flake#flake-references)

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If no `attrpath` then `default` is tried.

`github:Owner/repo#attrpath`
`git+https://github.com/NixOS/patchelf?ref=master&rev=f34751b88bd07d7f44f5cd3200fb4122bf916c7e`

can also reference flakes by just a filesystem path, ie:
`/home/jras/nixos#hieradata`.