These are the components that make up a King coilover shock. It's pretty obvious there's a lot happening in between the blue-anodized end cap and rod end. While most of the shock is filled with oil, the reservoir is partially filled with oil and partially with nitrogen. There's a dividing piston floating in the reservoir, separating the oil and nitrogen. As oil heats up, it expands against the dividing piston and the nitrogen which in turn compress into a smaller area. As oil cools down, it contracts and the nitrogen then expands to take up the "slack." In short, the reservoir's purpose is to keep shock action smooth and consistent.