Compositors are meant specifically for compositing other colors or maps. In image processing, compositing images refers to superimposing two or more images to combine them.
The Composite map type is made up of other maps, which you layer atop each other using the alpha channel and other methods. For this type of map, you can use overlay images that already contain an alpha channel, or employ built-in masking tools for overlaying only certain parts of a map.
With the Mask map, you can view one material through another on the surface. The mask controls where a second map is applied to the surface.
With the Mix map, you can combine two colors or materials on a single side of the surface. You can also animate the Mix Amount parameter, and draw map morphing function curves to control how the two maps are blended over time.
The RGB Multiply map is typically used as a Bump map, where you might want to combine two maps to achieve the correct result.
