PBR Material Library Issues.

Okay, so I've been experimenting with some small scale environment art to add to my portfolio, and in the documentation "XCOM2Mods_Envronments", the texture setup section refers to sampling from their "PBR Material Library" for the specular mask but I cannot for the life of me find out where it is. I've googled for it and searched for various things on this very reddit, but to no avail.

I realise I'm likely being a bit dense, but this is the first time I've used PBR materials and I'm not...entirely sure what I'm looking for(I'm a painterly dude before now). In the XCOM UE, there's a texture library, but I don't know what constitutes something specifically from the PBR Material Library, as it seems to have ALL of the textures in the entire game in that library.

There's also a catagory called "Physical Materials", but I can't see anything resembling textures in that catagory. It's just yellow boxes with "Physical Material" written on them.

Help a noob out?