The Basic Idea of the Composite Map

The idea is to be able to layer materials.

So we need a way to see through from one material to the next.

This is done with alpha channels or opacity.

So each material (except the base material) must have an opacity channel for this to work.

If you are doing organic or irregular material, you can make opacity channels with a gradient ramp set to diagonal with fractal noise.

Use the button marked A for additive opacity, this is what you would normally expect.

S is subtractive opacity.

The M button stands for Mix, which (oddly enough) is just like what happens with a Blend material without the mask.