Click this icon to delete the selected file sync rule(s). The following pop-up window is shown.
Note the checkbox Keep synchronized files on device, which is selected by default. If this field is not selected, all synchronized files comprised by the rule will be deleted from all affected devices when you delete the rule. This can have adverse effects, for instance if you have synchronized a folder to the [internal] root folder without specifying further directories. If this field was not selected, all files in the [internal] directory would be deleted.