Inner Edges Take a look at the top edge of the dial calipers. Those are to take measurements of the inside of holes like on the opening of a jar. Put them inside the opening and carefully spread them using the thumb grip. They will widen simultaneously as the jaws open. Bear in mind that hand made objects will have variance unlike the machine made objects of our time.
Depth Now look at the tail end of the dial calipers.
As you move the thumb grip, simultaneously as the jaws open and the blades spread, that rod will move out the bottom. That is a depth gauge which you can put into a vessel and measure it's depth. Open the calipers until you hit bottom and take your reading.