This document discusses control valves, which are used to control the flow of fluids in processes. It describes different types of control valves, including air-to-open and air-to-close valves which are chosen based on safety considerations. The document also discusses important control valve characteristics like gain, rangeability, and installed gain, and how these factors must be considered to ensure stable, fast control across a valve's operating range without needing frequent controller retuning. Valve characteristics describe the relationship between controller output and fluid flow through the valve.