I just got the details from one of our technical artists. I'll format it later for the wiki, but here it is already:
BM is indeed base colour/albedo
NM is our normal maps and we move our red channel to the alpha so rather than RGB we have XGBR this is to circumvent some compression details (red tends to get the least of the bytes)
PM is also indeed 3 combined grey scale maps with an optional alpha
R= Metal Mask
G = Roughness
B = Ambient Occlusion (actually unused we never got a AO slot in our shader model)
A= Optional reflectance value used for glass and unusual materials like some of the Eternal stuff
there are also 4 types of masks depending on their use
MSK1 MSK2 MSK MSKA
the number indicates number of available channels in the masks and considers 3 as a default in the naming
most FX maps are single channel with some occasional exceptions if colouring via the effect tool isn't sufficient
We use Block Compression
BC1 for RGB
BC3 for our normal maps and RGBA maps
BC4 for Single channel maps (R)
BC5 for double channel maps (RB)
