Now go over to Spencers photo and start painting over his nose. Instant plastic surgery. Kind of twisted, but potentially useful. Cloning from a channel to an alpha mask is often a good way to create nice alpha mask for images. Same goes for cloning from a channel to a layer.
Image means the source is either in that image or another drawable.
Pattern source means the currently selected pattern is used as the source. The difference and use of these two modes is demonstrated in the uses section of this documentation. Clone tool is set to, Align can have two different effects on the behaviour of Clone. This is useful for painting one section of an image to another. In Pattern Mode, Align has a different effect entirely.