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.
One file per server, in new format
  Each virtual server is backed up into its own separate file, but in a different format to the previous option. This new format is superior as it does not require a large file to be created in the `/tmp` directory first, this saving disk space.

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.