What are PostProcessInput1-6 used for?

My guess is that the input number indicates the pass in which the effect is computed, with PostProcessInput0 being the first pass and PostProcessInput6 being the last. I was going to look at the code but figured I’d post here first, and found others were interested at one point as well:

What are the use cases for PostProcessInput1-6? Why use them over PostProcessInput0?