An ICAO Aircraft Type Designator consists of not more than four characters and is used in flight plans and associated air traffic services messages. Each designator is, in principle, derived from the manufacturer’s product designation or from a commonly used military type number. Only one designator is assigned per aircraft type unless a variant has a difference in a performance that is potentially significant for air traffic services, or when no shared designator can be assigned for some other reason. Throughout SKYbrary, aircraft types / variants are identified using this designator. In principle, ICAO allocates a unique Aircraft Type Designator to all aircraft types /variants heavier than ultra/micro light.