This is a right-angle bracket for using between two sets of “rivet” shelves.  I had some space in between two of mine and thought I would make a bracket that would slot into the holes and allow me to mount an additional shelf in between the units.  The shelf sits on top of the bracket and can be fixed with a short drywall screw.  Tested with Vevor shelving but your mileage may vary depending on the brand, there are a lot of them out there.