The reference variable in a control or feedback control is a variable not affected directly by the related control or feedback control that is provided to the open loop control system or the control circuit from the exterior and that the output variable of the control or feedback control is intended to follow with the stipulated dependency (DIN 19 226).
Based on this definition, the reference variable is the same as the setpoint. In fact these two terms often overlap, e.g. in the steady state area on a feedback control. On the other hand they differ if, e.g., a ramp is moved along in the transition range. Then the setpoint adopts its value while the reference variable follows the ramp. Or: The setpoint for a temperature control is the constant room temperature. Due to changing outdoor temperatures this setpoint becomes the reference variable.
The reference variable can also be considered a setpoint that changes over time or changes a different variable.