Hi, I would like to expand a bit on to Heather's answer.
As you know, Rhino has no limit on layer nesting. That means that you can end up with quite a complex system of layers and geometry. We looked through some typical files and thought about how to best integrate this. What made the most sense was to generalize the layer system a bit. The main reasons for this was that as of right now Forma exposes only a handful of categories and the Rhino plugin's UI was getting quite busy. Instead of listing every single layer in a cramped long list, we only summarize the top two levels. That way you end up with something that's a bit easier to manage and still provides flexibility with the assignment.
Now this doesn't mean that the plugin ignores the rest of the geometry - if you look at the "select all" checkbox in the image below, you'll notice that even tho we have selected only a few layers, it summarizes that all 40-ish sub-layers will also be included. The geometry on those layers will inherit the parent layer's category and function (function assignment is only available for the building category).

Hope that clears things up.