Labels are created internally for device properties in electrical mode. These labels can be moved, deleted, and edited just as user-supplied labels. Once deleted, though, such labels can not be recreated except by recreating the device, or by using the !regen command. The underlying property is not deleted, it simply is not displayed in a label.
Labels can be edited by selecting the label before pressing the label button. If the label was generated for a property in electrical mode, the underlying property is also changed. This is a quick way to modify device properties, without invoking the Properties command button in the Edit Menu.