Calvin ✨

So traditionally you have three maps- albedo, normals and a third packed with metal, rough ness and AO (I call this one the masks map). Each of these three has 3 channels, R G B. That's nine channels.

Okay, so you can ditch the blue channel of the normal map these days as you can derive this in the shader. Z= sqrt (1 - saturate(xx + yy)) This gives you a free channel in your normal map.

So breath of the wild packs roughness into that blue channel. The logic here is they don't use AO, and unless a metalness map is needed, they just use two maps... albedo and normal (normal X, normal Y, roughness). Okay...

So if you are using PBR, you still need an Ambient Occlusion channel. So rather than pack roughness into the normal blue channel, it makes sense to put the AO channel in there- because material variants will change the albedo, metal and roughness but never the ao.