I do not know what multi pass rendering is, all I know is random sheets showing the same outline of an object but differently coloured while each own unique way of filling has a name like for instance "diffusion", "spectre", "volume", "shadows", "lighting" but there are many more and they seem to be freely combined within each sheet, they seem to represent kind of aspects of an object. I would like to know some more background info about multi-pass-rendering like its history, its purpose, its benefit, alternatives, the theory behind, the different kinds of what I called aspects.
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.
The basic idea is that you can control how the diferent passes interact with each other.
For example the original render was the top left image, but I wanted some dramatic caustics (the refracted light on the floor) so I made an aditional pass there, and another for stronger reflections.
Later you combine it for example in Photoshop.
Depending on the program used you can for example render a room with ambient light, and later with some artificial ligts turned on, etc.
There are a lot of aplications, another example, a type of rendering is called a deepth map, that computes how far an object is.
You can play with it to make "Fog"