This is the main tool for editing selections. You can simply use it to extend a selection from one place to another. The tool will automatically find edges. If it makes a mistake you can press Ctrl+Z to undo. The faster you drag, the larger the area it expands into. You can also change how large an area is selected by adjusting the Strength slider. If you select something by accident, you can undo and try again slower or with a lower strength. Alternatively, you can drag from another area in order to push a selection back if it has gone too far.
Because when you drag it selects areas all the way along where you drag, it’s impossible to select disconnected areas. However, if you click on a label, you can Lock it, which means you select that area anywhere in the image until you click again to unlock. You can click on labels on the image, or in the side panel. You can also lock labels by Shift clicking anywhere on the image that is marked as that label. This is the fastest way to lock areas as you often don’t have to move your mouse far to get to the area.