This cap holds down the mode dial lock button on the Sony a7Riii, a7Riv, and similar bodies so you can change modes easily with one finger. The hole in the center will accommodate a lanyard or loop of thin cord or monofilament.
It's a friction fit, and due to printing tolerances, it may be slightly too tight or too loose. If it's too tight, don't force it, just sand the inside wall a bit. If it's loose, add a small patch of thin tape to the inside wall.
The video below demonstrates the cap on an a7Riv, and for more details, see the longer video