This is one of the more newer maps that texture artists use. I often come across people that have never heard of such a thing so I will just explain it's purpose as simply as possible. You should open up the following image while reading this.
1. So imagine this is say 3% of your asset. 5 plains that share the same UV's and texture. It tiles but it sort of looks bland.
2. On UV channel1 the UV's are 5 boxes overlaying each other but in another channel (usually 2 or 3) we have UV's for multiple parts. We make them so small so there is room to have a unique face for each plain. This is to be the colour map and is usually 1024 or 2048 pixels.
3. This is a zoomed in view of the top left corner of the UV map. The rest of the map will be filled with all the other parts of the asset, I'm just not showing them here.
4. Very crude but hopefully it puts the idea across which is that you can paint unique details onto each plain.
5. So now all 5 plains do not look exactly the same.
The whole process really is just an alternative decal method. I almost always have colour maps now for every medium or large asset.
