B is the C common wire on this system.
You will need a Nest Learning model to handle 2 stages of heat and cooling.
Just make sure you know if you have a heat pump when you wire it up because the screw labels on this are "inner for non heat pumps" and use the "outer labels for a heat pump system".
Just one quick final check in here since activity has slowed down. We haven't heard from you in a while so we'll be locking this thread if there is no update after 24 hours. If you have any new questions, updates or just a discussion topic, feel free to start a new thread in the Community.