Please read these, and let me know if you need more info. I’m trying to improve the docs.
pivot input shift: Shifts the input value of the pivot point. Since contrast is highest around the pivot, this slider allows you to choose at which part of the input tonal range you want to have the most contrast.
pivot target output: Sets the target output luminance for the tone selected by the pivot’s input. This adjusts the brightness of the pivot point.
(dtdocs/content/module-reference/processing-modules/agx.md at agx · kofa73/dtdocs · GitHub)
In filmic, among the advanced settings, you can enable use custom mid-gray values, and then you get a slider on the scene tab that allows you to move the pivot point that gets mapped to mid-grey (in the output) lower or higher on the (input) exposure scale.
In agx, you can also adjust to what value that point is mapped (so the associated output value is not necessarily mid-grey). You have complete freedom is selecting the point of highest contrast, and can make brightness adjustments without modifying the black and white points.
(Compare and contrast: Filmic RGB vs AgX - #2 by kofa)