The Tree & Sky selection tool is available once you have dragged in both Tree and Sky labels. The tool is used to automatically find and label areas of the image as either tree or sky. If you drag over a complex area where trees are silhouetted against the sky, the trees will be automatically labeled tree and the sky automatically labeled sky. The area it looks for tree and sky in is determined automatically, but will tend to search in larger areas if you are dragging quickly. It is important to have good sky selections if you are making dramatic changes to the sky.