The Dial a Depth is the more traditional depth adjustment on most nailers. Turn the nut clockwise to shorten the trip and increase depth, turn it counterclockwise to extend the trip and decrease fastener depth. Using a lathe stapler, this is usually a set and forget feature.
The Power Control Valve is an air flow limiter. Set the dial a depth for what is considered a "normal" application for you with the Control maximized. If you then switch to shorter staples, or softer material, the Control gives you an way to reduce flow to allow for this change without having to readjust the Dial a Depth.