The keyfile which associates this machine with your tarsnap account. Create the keyfile with tarsnap-keygen.
Note that each individual archive (specified below) may also have its own individual keyfile specified. Tarsnap does not allow multiple concurrent backups with the same cache directory and key (starting a new backup will cause another one to fail). If you have multiple archives specified, you should either spread out your backups to be far apart, or specify a separate key for each archive. By default every archive defaults to using "/root/tarsnap.key".
It's recommended for backups that you generate a key for every archive using tarsnap-keygen(1), and then generate a write-only tarsnap key using tarsnap-keymgmt(1), and keep your master key(s) for a particular machine off-site.
The keyfile name should be given as a string and not a path, to avoid the key being copied into the Nix store.