The used Vello LHC-ET83D Dedicated Lens Hood is specifically designed for use with Canon’s EF 100-400mm f/4.5-5.6L IS II USM lens.
It is used to shade the front element of the lens to prevent lens flare, which improves image quality and contrast. An access window on the side of the hood allows you to adjust filters such as a polarizer without removing the lens hood. The hood mounts to the lens via its bayonet mount, and it also mounts in the reverse for compactness and storage. While mounted, it protects the lens from accidental impact.