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. Every time you add a new task to RsyncUI execute an estimation run and inspect the result before executing a real run. If you by accident set an empty catalog as source, RsyncUI, rsync, will delete all files in the destination.
Default parameters set by RsyncUI to rsync can be disabled task by task. If you decide to disable a default parameter, be sure you understand what the result is. A disabled default parameter can be enabled again.
Be safe
rsync is a fantastic tool, but used wrong may cause damage and lost data. There are some verification and checks in RsyncUI. But it is your own responsibilty to verify that new tasks in RsyncUI does exactly what you want it to do. Verify all new new tasks by a --dry-run, an estimate run. The snapshot feature of rsync is a very nice feature. In version 2.1.1 it is possible to change the snapshot number. If you by some reason want to change it, please be sure you know what you are doing and why you want to change it. If you change it, there are some cleanups to do before executing next snapshot synchronize task.
If you let RsyncUI do the job and you are most likely safe. RsyncUI is a free and opensource application, please read the MIT licence.
Aborting tasks
Please be aware it is an external task not controlled by RsyncUI, which executes the command line tool rsync. RsyncUI is monitoring the task for progress and termination. The user can abort a tasks at any time. Please let the abort to finish and cleanup before starting a new task. It might take a few seconds. If not RsyncUI might become unresponsive.
RsyncUI as your main tool for backup
RsyncUI is not developed to be an easy to use synchronize and backup tool. The main purpose is to assist and ease the use of rsync to synchronize files on your Mac to remote FreeBSD and Linux servers or to a local attached disk. And of course restore files if required.
The User Interface of RsyncUI can for users who dont know rsync, be difficult or complex to understand. The main objective is to ease the use of rsync, not teach macOS users how to use it. That is beyond the scope. Setting wrong parameters to rsync can result in deleted data. And RsyncUI will not stop you for doing so. That is why it is very important to execute a simulated run, --dry-run, and inspect what happens before a real run.