The U1 terminals control either ventilation or humidifier/dehumidifier. You need to trace the two wires and see where they go.
I installed the Nest and didn't use brown and black. It has an orange wire (O/B) that is for the heat pump. I have a basement which is a dual zone with the 1st floor and the upstairs (heat pump thermostat) which is a separate zone. The app tells me that the Nest is incompatible with my system. Does this sound right?
@jcampagna42, we wanted to follow up and see if you are still in need of any help. Please let us know if you are still having any concerns or questions from here, as we would be happy to take a closer look and assist you further.
I appreciate the help, Jake and Patrick_Caezza.