Particular PBR Maps

I'm sure this has been answered before somewhere, so I apologize in advance, but I couldn't find it. Obviously, Unity's standard shader can use a Metallic or Specular setup, because PBR uses a Metallic or Specular setup. Again, obviously, there are certain maps the are common to both setups, and there are maps specific to each setup.

Image courtesy of Allegorithmic

I see all the slots for the common maps in Unity, but there are 2 maps specific to each setup. This is why people often call the two setups metal/rough and spec/gloss. I once again apologize for telling a lot of you what you already know, I just want to be absolutely sure we're on the same page.

Now here is what I don't understand. If there are two maps specific to the spec/gloss setup, where is the slot for the gloss map? If a metal/rough setup has a metallic map and a roughness map, where can I put the roughness map in Unity? The best I see is a smoothness slider, which would be extremely limiting for obvious reasons, so where do I put these particular maps? Is it embedded in the metallic and specular maps? If so, how would I go about combining the two? Thank you in advance