In a texture mask, what channel should be what information?

I’ve always been wondering and I’ve got several different answers. My question is quite simple and It’s just what the title says.

I noticed the starter content has several different masks, such as the T_Brick_Clay_Beveled_M mask.
Which channel in this texture represents what? I’ve heard that:

R = Roughness
G = Ambient Occlusion
B = Metallic

Is this true, and is this the recommended “standard” to follow? Also what is the actual name of this type of mask? I refer to them as a MRA-masks, but I can’t really find anything searching for that.

It doesnt matter which gray-scale file you put where, as long as you and the team you are working with all adhere to it.

There is no specific word for it, because it can be any kind of grayscale texture or mask.
A roughness texture isnt a mask, neither is AO. Metal " could" be considered a mask.

It could also be any other black/white or grayscale texture.
So you could probably call it nested/grouped/merged textures.

There is no standard. Some people create a standard of their own while some use naming conventions and others just use them willy-nilly.

If you need it to be obvious for its use (like in a team environment) then you could go with a naming convention, something long and painful to read like:

Example of naming for per channel masks:

Example of naming for per model area masks:

Really do whatever you like. As for how that specific texture was used you just have to either look at the material it is used in or suss it yourself by applying each channel and seeing how it fits. Remember, some masks are for channels while others may be for assigning to locations on a mesh.