The Albedo Map (which can contain 1 or 2 textures - Base Color (RGB Channels) and/ or Smoothness (Alpha Channel).
Albedo Map: Contains the base color texture for the material.
Metallic Map: Map that determines the metallic nature of the material. The metallic map must also contain a smoothness map and potentially an ambient occlusion map. These need to be packed in the following channels: R = Metallic, G = Occlusion, B = Empty, A = Smoothness.
And then it says:
Smoothness Properties - Smoothness Source
MetallicAlpha The smoothness map is located in the alpha channel of the metallic map.
AlbedoAlpha The smoothness map is located in the alpha channel of the albedo map.
So it does seem like the smoothness can be in either one of two files. But... there's nothing at all about the 'normal' alpha mask of the albedo (diffuse) that defines opacity? So here's two more questions:
1) why would one want to put the smoothness channel in the albedo's alpha channel?
2) If you do put the smoothness in the albedo's alpha, then where do you put the 'normal' alpha mask/opacity? Normally the albedo's alpha is where you put that information? I ask this question all the more so because of this passage under "Masked properties":
Use AlphaToCoverage Uses the alpha channel to determine if the masked section should be visible or not, allows for anti-aliasing support on masked materials. You will still want to set a masked threshold to use for shadow support.
It just doesn't say which alpha channel it looks at?