I was trying to look up what a Transmission Color pass was in the Blender docs, and I only got this:
Same as above, but for transmission BSDFs.
This was pretty vague, and I didn't understand what transmission was, so I tried a test scene. With the default scene in Cycles Render, I added a plane scaled by 5 at (0,0,-1). I moved the default point light at (-0.7, -5, 6). I changed the default cube's material to a Glass BSDF (I guessed transmission had to do with an object that's translucent), and I selected the "Color" Option in Properties -> Render Layers -> Passes -> Transmission to use the Transmission Color pass in the Compositing Tab. I rendered the scene, hooked up the "Transmission Color" output to the Compositing node, and got this.
I still didn't understand what was going on, so I tried to google "transmission color pass", and I found a site that said this:
Transmission takes place when light passes through an object without being essentially changed
I also got this from 1:02 in this YouTube video about what happens to light in transmission:
It can be granted safe passage through the medium itself. Sometimes it'll be refracted...
So from all of this, I'm guessing a Transmission Color pass is a pass where white parts show areas that allow light to pass through freely and black parts show areas where no light passes through. I'm not entirely sure on this, so I was wondering if someone has a different answer as to what a Transmission Color pass is. Also, does this pass only output a black/white image, no gray/color?