Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General PBR materials workflow understanding in DOS2

I believe PM stands for "Physical Map", i.e. a texture for all PBR parameters.
The R,G,B channels might be MetalMask, Roughness and Reflectance.
(Speculation based on the shader code in Shaders\DX11\PBR.shdh)