This field allows the compression type to be overridden for this backup. If not set, the global default format (set on the Virtualmin Configuration page) will be used.

By default, Virtualmin uses `gzip` compression on its backup files. However, this option can be used to select the space efficient (but CPU-intensive) `bzip2` format instead. You can also use un-compressed `tar` format backups, which may be useful if you want a speedy backup, or if you are writing to tape. Or the `zip` format, which is easier to extract on other operating systems.

This option is only used when backing up virtual servers to a directory, with one file per domain. When backing up to a single file, the filename determines the compression format (like `domains.tar.gz` or `domains.tar`).