The texturing of rigblocks is a complicated process since the hand authored texture layers are then blended with the engine SKIN PAINT system to produce the final result.
Our textures are kept in PSD format, and consists of 8 to 9 extra channels. There is a strict naming convention of the channels since it governs how the layers are exported by our asset exporter.
The diffuse alpha mask controls how much of the diffuse component of the procedural skinpaint (picked by the player) will blend in with the hand authored texture.
The spec and bump mask controls the blending of the respective components with the skinpaint as well.
The tint layers control how much USER-picked color will be hue-shifted on the diffuse texture. The player has control of 3 colors, and thus we have three tint layers for each color.
For more information on how the Skinpaint system worked, visit Ocean’s post.