Unique Calibration
The Emissions/Corporate Average Fuel Economy/CO2 Compliance Department is responsible for assigning these calibration numbers. Unique identifications are assigned to cover similar vehicles to differentiate between 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 identify a 4-valve engine provides an easily identifiable difference.