In addition to passenger duties, the A300 became widely used by air freight operators; according to Airbus, it is the best selling freight aircraft of all time. Various variants of the A300 were built to meet customer demands, often for diverse roles such as aerial refueling tankers, freighter models (new-build and conversions), combi aircraft, military airlifter, and VIP transport. Perhaps the most visually unique of the variants is the A300-600ST Beluga, an oversize cargo-carrying model operated by Airbus to carry aircraft sections between their manufacturing facilities. The A300 was the basis for, and retained a high level of commonality with, the second airliner produced by Airbus, the smaller Airbus A310.