The replaces the existing selection. The alpha channel itself is not changed.
command creates a selection in the current layer from the alpha channel, which encodes transparency. Opaque areas are fully selected, transparent areas are unselected, and translucent areas are partially selected. This selectionThe other commands in this group of operations are similar, except that instead of completely replacing the existing selection with the selection produced from the alpha channel, they either add the two selections, subtract the alpha selection from the existing selection, or create a selection that is the intersection of the two.
You can access this command from the image menubar through
→ →or from the pop-up menu which appears when you right-click on the active layer in the Layer Dialog.