The Output value is a temperature, given that Input and Setpoint are both temperatures, isn't it?
No, the output represents a unitless value that represents an error term to be applied to whatever is used to try and manipulate the process being controlled to try and force the process variable (the input) closer to the setpoint value. At our refinery most all the P&ID loops had unit values for the inputs and setpoints, but outputs were always scaled at 0-100% and would typically represent the value to be sent to a control valve where 0% might mean fully closed control valve and 100% would mean fully opened. For a temperature control loop the output might be manipulating a steam flow control valve or possibly an electrically powered heating element, so again the output is best expressed as a simple value 0-100% of output control range. Any other units of measurement makes no sense for the output term.
Lefty