This is an excerpt from the Device Extension Wiki site for the new Thermostat Wrapper device, if you would like to get all of the details be sure to check out the Wiki for more details.
This is released fully into the Plugin Store but this device (not the release) is still experimental. While it is being regularly enhanced, more features are forthcoming, I encourage you to try it as-is and let me know what else you might like to see.
Device Extensions is getting some upgrades from just being conversions and extensions, wrappers are now being added and the first of the bunch is the Thermostat Wrapper. A wrapper is just as the name implies, it wraps around another device to either modify the default behavior of the device or to add functionality that wasn't there before. It can also be used as a "virtual" device that can have each function assigned to a different device or action group.
The wrapper concept is that you can optionally select any thermostat in Indigo to wrap (you don't have to actually wrap anything) and any function or value that you do not explicitly override will simply pass through to the selected thermostat. For instance, if the only function you decide to override is the current temperature and humidity then those values will be retrieved from whatever devices and states you specify for them while the rest of the thermostats data and functions will pass through.
This was primarily designed as a way to get temperature sensor data to pass through to HomeKit since Homebridge-Indigo/Homebridge Buddy does not yet support independent temperature sensors. While it shows in HomeKit as a thermostat, the desired result was achieved since the thermostat reads the temperature from the sensor.