I don’t follow. Are you asking what PBR is?
The very essence of it is the law of energy conservation. At the very basic level it means that reflected light can’t have greater intensity than the light source that shines upon it. So: incoming light = diffuse reflection + specular reflection. If you have a specular surface that means the diffuse reflection will be dimmer. If you have a really rough surface (that is approximated in UE 4 with the roughness parameter in materials) it means that the diffuse reflection will be more intense while the specular has to be lower because the sum of those two can’t be more than the incoming light.
That wasn’t how lighting in UE 3 behaved for example. In UE 3 you could make the specular intensity arbitrarily high and it didn’t influence how the diffuse reflections behaved.