The System Difference Code (SDC) is intended to identify different versions of a system. To be valid it must be between one and four characters long which can only include numbers and upper-case letters. Once the model identification code had been selected, choosing a system difference code was found to be relatively easy. As they MIC identified internal airline documents it was natural to use airline divisions for the SDC. As the migration project only covered the engineering part of the business a code that signified this was chosen.