Zone Humidity is the default sensor. If you use this option, you may also add humidity as needed during unoccupied.
Return Air Humidity is another option to use for control.
Discharge Humidity High Limit Control can be used to add an analog input to monitor the discharge air humidity and override the humidifier signal to prevent condensing conditions in the duct work.
Adding a humidifier requires selecting the sensor to be used for the control loop.