As has been discussed many times already, selections, channel masks, layer masks, and alpha channels are all different implementations of the same thing. They partition a layer into selected pixels, partially selected pixels, and unselected pixels. Although they all do the same thing, they do it in slightly different ways...ways that are remarkably complementary.
Being able to convert a selection to a layer mask or an alpha channel
to a channel mask is often useful, however, it may not always be
obvious how to go about it. As an aid to the reader, the table shown
in Figure 4.35