If you are new to the command line tool rsync and RsyncUI please read this information. RsyncUI is a GUI only on top of the command line tool. It is rsync which does the actual work, not RsyncUI
The –delete parameter and new tasks The --delete parameter is a default parameter set by RsyncUI to rsync. The parameter instructs rsync too keep the source and destination in sync. The parameter instructs rsync to delete all files in the destination which are not present in the source.