This is the main tool for editing selections. Click on an area that has been correctly selected (such as an area of Sky) and drag to extend the selection from one place to another. The tool will automatically find edges.
The faster you drag, the larger the selected area will be. 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.
Tip: Undo a change made during the selection by pressing Ctrl+Z (Cmd+Z).
When you drag it selects areas all the way along where you drag. This means you cannot selected disconnected areas simply by dragging.
To apply the same label to areas which are not connected, click on the label Lock it. Now any area you select using the tools will have this label applied. Click on the label again to unlock it.
You can click on labels on the image, or in the side panel. You can also lock labels by holding Shift while clicking anywhere on the image that is marked as that label.