So, there are some great tutorials around the web about isolating render passes in Cycles so that you can make changes, and then recombine all of your work as one complete image.
You'll probably have a Compositor node setup that looks something like this:
Blender's Manual used to have a schematic explaining how these can be recombined:
But not all of the pass types are explained by this deprecated schematic, as now there are more pass types (SSS, Shadow...)
What would be the "master" routing, so that if all passes are separated out they would all get recombined properly to form the complete image?
I set off on a mission to create such a routing, but realized I wasn't sure how to recombine some of the passes.