This isn't a new layer so much as a fork of the old A Roof layer.
In AC9, as everyone knows, model polygons can have a cover fill. This means generally much less drawing, and more showing of those model elements in plan. It also gives the opportunity to 'stack' model elements in plan using Display Order (Send forward/back.)
In AC8.1 and earlier, you might have placed a counter slab on a A Cabs3, and a fill on A Cabs2, now you can have just the slab on A Cabs2. This promotes unity.
This ability, of course, extends to roof elements. It is possible, with some work, to show a roof plan built from actual roofs. This technique will have its own post.
Where we used to have one layer for roofs, because they were all 3D-only, now we have two, so we can choose to show roofs in plan, or not. We maintain the pattern of other 2D/3D layer pairs: Deck2/Deck3, Cabs2/Cabs3, Stair2/Stair3, etc.