Unique Calibration
WARNING: This page is about a different car, the 2014 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.
The Emissions/Corporate Average Fuel Economy/CO2 Compliance Department is responsible for assigning these calibration numbers. Unique calibration identifications are assigned to cover similar vehicles to differentiate tires, drive configurations, final drive ratios and other calibration-significant factors.
These 2 characters are chosen by the analyst to provide identifiable information unique to each calibration. For example, using the number 2 to denote a 2-valve engine versus using the number 4 to denote a 4-valve engine, offers an easily identifiable difference.