I know that @randy knows. He is being a good host and asking the layman questions to help me frame this topic more clearly 
The Beauty Pass is the preferred result of the CG render. On its way to being completed the renderer will do different passes/calculations, and it will combine them all together and deliver the result that the CG artists intended.
You can save off these individual passes, called AOVs ( Arbitrary output variables) and recombine them to rebuild the Beauty Pass.
Why would you want to do this? Well I use the example of a render of a red leather wallet. Your mobile phone client thinks that the red is a little too Vodafone and wants you to grade everything that is red to blue. You haven’t got time to go back and render again so you have a go at grading it in comp.
With only a single render pass you are going to be dealing with the shiny red wallet and when you are grading it you don’t want any of the specular (shine) or reflections to change colour. If you rebuild the CG beauty pass you have access to all of the AOV passes that went in to the render and by changing the colour (diffuse pass) before adding the specular (shine) and then the reflection pass means you don’t have to worry about messing up the colour on those.
It helps to know how different renders use different AOVs and how they are combined to make the beauty. I deal 90% with RedShift and 10% Arnold.
This came up in a discussion on discord about Subtractive CG compositing which @finnjaeger talks about in this LOGIK Live
