almost certainly to the factory tie-down rings. They're usually in those corners on most SUVs.
I'm not sure why folks do that, especially on self-built systems. It sacrifices a good bit of cubic space and leaves a gap on both sides of the storage platform. Just as easy to remove the factory fittings and make use of the mounting location to directly bolt the bottom of a storage platform into the vehicle. That way the storage platform or drawers can be full width. Cheaper than turnbuckles, too.
I've got my MWC carrier bolted down in the back of my suburban like that now, and that's how I plan to do my 2-drawer platform. Just had to get a longer bolt to reach the factory hole, a fender washer, and drilled a slightly larger hole in the bottom of my carrier for slop / error in transferring the measurements.