http://teanmoon.blogspot.com/2016/04/the-specular-texture.html
What is it? The specular is what controls the shininess of your mesh in game.
You can find it in the warehouse tab. The CAS specular is the RLES texture and for objects it is one of the two DST images.
You can export these textures either as .png's or as .dds's. DDS textures allow you to control each channel individually. PNG textures show the RGB channels individually like a DDS, but the strength of the specular is dependent on the opacity of the image instead of the color of the alpha channel.
If you are looking at the HSB values for a color in PS, the B value tells you the strength.
The specular made of four parts for objects and five for CAS items:
The red channel: This control the "cube map blur." This channel adds glossiness to your texture.
Both boxes have the specular strength set at 50%, but the box on the left has 50% red, whereas the box on the right has 100% red. The texture on the right has a slightly more blurred out shine.
50% red specular color
100% Red specular color
The green channel: This channel controls the "Cube map" strength. This gives your object a 3D-shine.
The specular strength is set to 50%. The left box has 50% green, the right box has 100% green. The brightness of the shine pattern is much stronger on the right box.