Physically based rendering from first principles

The discussion of the difference between metal and non-metal is a bit unclear. When he says that metal has no "diffuse reflection" but non-metals do, I think what he might mean is that metals have no subsurface scattering. Rough (not mirror-like) reflections do certainly also look "diffuse".

Yes, in rendering when we talk about “diffuse” reflection we implicitly mean fuzzy reflections due to light penetrating the material and exiting a small distance away from the entry point (less than a pixel far)

The term subsurface scattering is used for light that exits further away (more than a pixel) like in simulations of human skin

However that part of the article isn’t clear enough and warrants a rewrite