Trane AC model numbers have nine identifying letters and numbers, with each place in the line signifying a specific piece of information. For example, the first spot in the model number identifies the AC unit's brand, and the last spot is the service digit.
Trane AC model numbers denote the AC unit's furnace configuration, with a U meaning upflow and a D meaning downflow. The model number also gives information about the model's design changes, with the fifth spot in the number denoting major design changes and the eighth spot describing minor design changes. The sixth spot in a Tran AC unit's model number describes the power supply and fuel.