i was confused by this too when i received my first backplate and tried to set it up. you've got backplate, then the one-piece harness, then wing harness, then STA. none of the bolts that came with the kit would fit because there were so many eyelets and harnesses stacked on top of each other that it was just too thick on the top hole. the bottom hole is no problem because it's just BP, wing harness, and STA.
not sure if it's correct, but what i ended up doing was attaching the one-piece harness to one hole using one bolt, then attaching the STA and wing to the backplate on another hole that's higher up and separate from the one-piece harness hole.