The “material ratio curve” tells us how much material we encounter (as a percentage) as we slice through a surface at various depths. You may also see it called the Abbott-Firestone Curve (based on the two engineers who developed it) or the Bearing Ratio Curve.
The Material Ratio Curve tells us about the peak, core, and valley regions of a surface. We can also look at the material ratio curve over time to understand how a surface wears—an indispensible tool in applications such as engine bores and cylinders.
We have quite a bit of information available discussing the Material Ratio curve and the related Rk Parameters:
The Surface Texture Answer Book dedicates many pages to the topic as well.