Texture Maps are specified through the Material Editor and cover a wide variety of options. The available texture slots are often dynamic and depends on which shader is selected and which shader options are activated.
Decal Opacity Map
The decal slot has different functions depending on what kind of shader is used. It is activated with the Illum and Cloth shader.
Decal for the illum shader:
The Illum Shader is linked to the glow feature. Use it to add animated glow to your material without changing the diffuse map.
First, add a glow texture to the decal slot in the material editor. Then add a glow amount value to make the effect visible. Now you can see the glow texture on your model. You can use both black/white and color glow textures.
The glow texture can also be animated. To do this, set up the oscillator in the decal slot sub menu.
To make the decal glow texture visible on specific areas, paint an alpha map for it. Save the alpha map in the alpha channel of the diffuse map. White represents the area where the glow will be visible, and black will hide the glow.
Here is an example. The area where the glow is visible is controlled by a texture stored in the alpha channel of the diffuse map:
Decal for cloth shader:
In combination with the cloth shader, you can occlude or illuminate your model by using an occlusion map as a decal. An example of this is adding dirt to cloth.
You can use any RGB texture. It will be blended into your diffuse map with a mode that is comparable to Photoshop's hard light blend mode.
The decal used will be tiled the same way as the diffuse map is tiled. If you change the tiling of the diffuse map, the decal tiling with also be changed.