By default, Virtualmin backups will include all files in the selected domains' home directories. However, if you regularly backup large virtual servers, a lot of time may be spent copying files that have not changed recently.

If the Incremental option is selected here, only those files that have changed or were created since the last full backup will be included. This is most useful when you have two scheduled backups, such as a weekly full backup and a daily incremental. Assuming that most files in a domain are un-changed day to day, this will substantially reduce the size and time of daily backups.

Be aware that when restoring, you will need to restore first the full backup and then the incremental to recover all files. Also, other data such as Apache configurations and databases is not incrementally backed up, and so will always be included regardless of this option.