If Selection is active, actions such as painting or adding filters will be applied only to the parts of the Terrain height field that are within the selected area.
Select All - Puts the entire elevation grid in the Selection buffer. This is represented as a rectangle around the entire Terrain Preview window.
Deselect - Flushes the Selection buffer.
Invert - Reverses values in the Selection buffer. Height field segments that were not previously selected will be selected, and vice versa.
Feather - Blurs the Selection edge. This feature enables a smooth blending between the selected area and the surrounding Terrain.