A control law is a mathematical formula used by the controller to determine the output u that is sent to the plant. In a feedback control scenario, the output u can lead to robustness to uncertainty, and can be used to design system dynamics. A control law, which is usually part of the controller, takes the output of a process one wants to control as input. This input is then used in an algorithm that the controller uses to determine u, which is the input to the system (plant). For example, in a cruise control system, the control law compares the reference speed to the current speed and uses this error (vr - v) in a PID control algorithm to determine the signal u that is then sent to the actuators to decrease the error between the desired and actual speed.