Our current licensing scheme is tied to hardware IDs from both host system and guest system. This means that migrating virtual machine to a different host system requires new license. No license is needed for offline backups. Only running images need a license.
In order to minimize downtime, you can ask for a backup license for a second server. You need to select one backup server, because it is not possible to create universal backup license for arbitrary hardware ID. Please ask at firstname.lastname@example.org for the price of backup license.