Trying to figure out what this U1 wire is used for in my HVAC system.

I'm looking at my thermostat (Honewell thermostat) that connects to a HZ311 zone controller. The only place I can find the brown U1 wire near the controller is going to what looks like a damper. Basically I'm trying to see if I can upgrade to a smart thermostat, but without knowing what the U1 wire is (I don't think I have a dehumidifier) it is difficult to know what thermostats are supported. My house was build in 2012. More photos at the following link https://imgur.com/a/WM4US Anything I should be looking for that would help in my hunt? Thanks

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