In the fictional, comic book version of the Sunspot, Below Decks is the network of corridors and rooms that line the hull of the spinning cylinder that contains the Garden. It contains cities of people, most of the population, along with a mass transit system.
It is many layers thick, with, with the lower decks closer to the outside of the hull. The major populace occupy the first two top decks, just below the surface of the Garden. Below that are the fallow decks, kept mostly empty and clean, waiting for population growth.
In the story, there are dissenters known as Monsters, who live apart from society. They are allowed to make their homes in the fallow decks.
We don't have an analog for any of this in our system's inworld. We never dream about it, and we are having a hard time imagining it as we work on the comic. This is probably because there's nothing like it in the outerworld for us. Not in our own life. So, we've extrapolated the idea from science fiction concepts from other stories, but we haven't really built it into our own headspaces. At least, not to a degree that we've encountered it yet.
Essentially, in our inworld, this area is probably labeled "Under Construction". And we are actively working on visualizing it, so that we can draw it in the comics.