Difference between "backup passphrase" and "backup key"?

The documentation about backup and restore makes a point of distinguishing between “backup passphrase” and “backup key”.

The OnlyKey desktop software (latest version, v5.3.0) seems to only allow to set a “backup passphrase”: Under “Setup” → “Set Backup Passphrase or Key”, one is queried only for a “passphrase”, and the accompanying instructions only talk about a “passphrase”, not a “key”.

Hence my questions:

  • What exactly is meant by “backup key” in this context?
  • Why is there no option to set a “backup key” under “Setup” → “Set Backup Passphrase of Key”?

When doing initial setup the default is a backup passphrase, however you can also use a PGP key instead if you select option to do so.

Is this the same key that can be set under “Advanced” → “Add Private Key” (when “Set as backup key” is checked)? If so, can this be done at a later point (i.e., after initial setup)?

Right, after initial setup you could change the backup key by loading a private key, such as on the keys tab and checking set as backup key.