A passphrase for an SSH private key (as a Systemd credential name), usually provided on generation of the key with rad auth.
If set to the default value of null, radicle will optionally import the passphrase from a credential named xyz.radicle.node.passphrase.
If the passphrase is not set, radicle will prompt for it.
If configured as a credential name it will be imported via ImportCredential= in the service configuration. Refer to the systemd-creds documentation for more details https://systemd.io/CREDENTIALS/