What is a multipurpose map? What does it do? How do you make one?

Also I have noticed that those doors on The Pillar Of Autum have a light on them in the multi pupose map where the light is green when the door is unlocked and red when closed. How does this work. I know there is something to do with chanels.I am making a door just like the a10 one, but obviously it will look different.

ok, the multipurpose map is used to add things to the diffuse texture.
The multipurpose map is works based on channels - red, green, blue and alpha
You can read up on what each channel controls in any shader_model.
Red=shine
Green=glow
Blue=colour change
Alpha=colourchange mask ( i think)

The glow that is added in the green channel of the MP map, the colour of that glow is controlled by the colour ( in same place) on the diffuse map. I think there is also a section to maybe set the colour of that glow but im not sure.