Specular and metallic map texture channels in the standard shader

What is each channel doing in specular and metallic textures?
As an example in a normal image R=red G=green B=blue A=alpha
in a heightmap Black= lowest point white = highest point
I can’t find any documentations regarding this 2 texture maps, I know they are different but what is each channel doing? Thanks!

When you use Diffuse-Specular-Smoothness setup(the ‘Standard(Specular setup)’ shader), the layout the of the Specular map is: Specular Intensity(RGB) Smoothness(A). (Note that because of energy conserving, increasing smoothness may usually result in brighter specular.)

When you are using BaseColor-Metallic-Smoothness setup(the ‘Standard’ shader), the layout of the Metallic map is: Metallic(R) Smoothness(A).

Alpha channel is ‘smoothness’ in both case.

In BaseColor-Metallic-Smoothness setup, specular intensity (and ‘diffuse’) is actually computed from BaseColor and Metallic. That is to say, the ‘metallic setup’ is internally converted to ‘specular setup’ before lighting calculation.

I think the Metallic setup is more intuitive though.

After some tests the specoular map seems to use RGB as color and A as intensity while the metallic map uses R as how metallic is the material an A as smoothness, though I’m not 100% sure