Airport Codes will be unique. There is an ISO coding system for most Airports but there there stills needs to be the ability to create our own if no ISO code exists. The system should auto generate a code if the user does not define one. If the code is changed at a later stage, the system should automatically reflect the change as everything should be tied to the ID. So if the system generated a code 123 and this was later changed to ABZ then everything that was 123 would change to ABZ automatically.