In certain scenarios, there may not be a cable path to an area that a RTU or AHU serves, or it may not be in an architecturally desirable location for wall sensors. In this case, it makes sense to use the return air as your space temperature reference.
Example: An AHU serves a large open lobby space with marble walls. The architect does not want anything mounted to the walls.
Install a HyperStat at or near the unit, install a 75F Duct Sensor, and set the HyperStat to operate off of a remote Temp/Humidity sensor.
The AHU will have an excellent reference for space temperature and humidity while eliminating long cable runs and making your Architect happy!
Remember to install your Supply Air Temperature sensor on TH1!