Backup format
This option controls the file format that Virtualmin will use for this backup. The available options are :
Single archive file
All virtual servers are storing in a single file, in the location specified by the Backup destination.
One file per server
Each virtual server is backed up into its own separate file, in the directory specified by the Backup destination.
The final option is recommended, as it allowed individual virtual servers to be more easily restored, and saves on temporary disk space during the backup.