Case-back is for keeping the movement inside the case, preventing dusts and water. It’s mostly stainless steel with three ways to assemble:

The case-back is assembled with the case directly so it is worse waterproof than others.

The case-back and the case both have threads to assemble together so it is better waterproof than others.

Assemble case and case-back with screws so it is better waterproof. The appearance of it could be categorized in seeing through the movement or not; if you can see through the movement, the case-back is usually built with blue diamond glass.