No, it would be for regular surface materials. The idea with using material based fog is that you render the fog in the pixel shader of each drawn scene object. When just using depth it’s a very cheap simple static fog.
You could extend it with more material code to do more advanced effects like varying colors, height based layers and maybe even simple animations. It is calculated for every pixel in the scene though, so the cost can add up, especially if you sample textures in it.