Tripod socket
Some longer Sigma lenses have a tripod socket attachment. Usually the socket is included with the lens, except on the 100-400mm F5-6.3 DG DN OS | C, on which it can be purchased separately. Some tripod feet have an Arca Swiss type shape, so can be attached straight on to a compatible tripod. The purpose of a tripod foot is to ensure the lens is properly balanced on a tripod. Without one, a tripod plate would need to be attached to the camera, which when used with a long, heavy lens, would place a great deal of strain on the lens and camera mount.