That aft void on the Excelsior model seemed like a cargo/ storage access point. I'd even suggest that there could be a roll down door that we haven't seen because it wasn't required. But the only point to that may be that because it would be only really accessible by the decks above, I'd guess that the storage would be for volatile equipment that is meant to be deployed at some point. Or perhaps aft sensor/ communication platforms that just weren't meant to be accessed except for odd maintenance.