Another layman here. In claim 1 it refers to "a space outside the structure" which to me implies this is literally a hold right through the aircraft. EDIT: I think I'm wrong here, the structure refers just to the cabin.

Figure 2 shows it without the aerodynamic envelope. Figure 8 shows the cross section. There are hatches on the bottom which open up for loading and unloading. These can be used for stiffening, but don't need to withstand pressure so can be weaker.

In paragraph 0085, this interior unpressurized space can be used for luggage or freight. Or (paragraph 0094) to store the spiral staircase.