On mine the its where the outside speakers and bright docking lights are mounted not to mention the upper mounts for the care free awning. It also has a rain gutter across the back. the whole shelf seems to be designed to keep rainwater from running down onto the back door
2011 F350 6.7L Diesel 4x4 CrewCab longbed Dually, 2019 Lance 1062, Torqlift Talons, Fast Guns, upper and lower Stable Loads, Super Hitch, 48" Super Truss, Airlift loadlifter 5000 extreme airbags