So after reading this Unreal Engine 4 Documentation page: https://docs.unrealengine.com/latest/INT/Engine/Rendering/Materials/LayeredMaterials/ (Which describes how to use material layering in UE4)
I wanted to know how material layering is implemented. (Notice how the 'unreal engine' part of the rocket has different material and aswell as the scratches on the rocket?) Is it done by adding together the 2 final variables that contain the color's for each respective material after being processed by a BRDF?