How does metahuman's composite texture for roughness work?

I guys, I’m trying to learn metahuman’s and its material editing.
Of the body’s material, I found that metahuman doesn’t have a roughness texture, instead, it uses a thing called composite texture from roughness map.

Composite Texture | Unreal Engine 4.27 Documentation
This document even does not have a 5.2 engine version.

The problem is: I cannot repeat this trick on metahuman’s texture.


If you disable the compositing texture for metahuman’s body roughness, you get a blank black map. Since the document implies that you need to have a roughness map as the base and then composite normal detail into it.
How to explain this?