Maybe you read a reference to something very specific, but my guess is that a dust layer can refer to many things. In general it’s a layer that is added to generate some sort of noise or something to add imperfections to your shot. CG is very clean by default and the real world is full of imperfections. So my guess is that you’ve read about people creating passes that include scratches, dust, noise, etc. to make the surfaces of their objects just a bit imperfect to add to the realism of the render.
If floating dust is something you are looking for, a good trick is to just create a group of triangles and color them a general hazy brownish color, maybe have them rotating and moving slowly through your scene. It’s nice to make these objects shiny so they’ll catch your lighting on occasion. Render this pass seperately, composite it and blur the hell out of it so it’s unrecognizable. This can be a nice way to add a bit of depth, and imperfection to your scene.
So in general, I’m guessing that your dust pass is just a pass to add imperfections. I could be completely wrong on this though so if someone knows otherwise, please correct me.