This is from Centralite support:
The "2" refers to the second stage of either your heat or cool. The second stage is called for when the desired setpoint is more than six degrees different from the current temperature. If your unit has a second stage, it will engage, speeding up your heating or cooling until it reaches that six-degree threshhold, at which point it will move to the first stage, and the "2" will disappear.
If your unit does not have a second stage, nothing will happen. You'll need to drop your setpoint to within the six-degree limit in order for the first stage to engage.