Attached Link: http://aprilgem.com/log/archive/2003/07/12.html

Save to an alpha channel, yes. Then, when you want to attach it to a specific layer, load the selection (or activate the alpha channel) first, select the layer you want to mask, and then click the mask icon in the layers palette. There are other ways to create layer-specific masks. I'm linking to one of my tutorials where you can learn another way.