The Add to Selection command creates a selection in the current layer from the Alpha Channel. Opaque pixels are fully selected, transparent pixels are unselected, and translucent pixels are partially selected. This selection is added to the existing selection. The alpha channel itself is not changed.

The other commands in this group of operations are similar, except that instead of adding to the existing selection with the selection produced from the active layer, they either completely replace the selection with a selection produced from the alpha selection, subtract the alpha selection from the existing selection, or create a selection that is the intersection of the two.