BC, Aztec also is used by Deutsche Bahn, Trenitalia, Nederlandse Spoorwegen and by Swiss Federal Railways for tickets sold online and printed out by customers. An Aztec code barcode is used by Heathrow Express and National Express East Coast delivered to mobile phones and displayed on their screens, and on self print tickets. The barcode is scanned by a handheld scanner by on-train staff to validate the ticket. The scanners in Germany do not have internet access.
From what I heard, the airlines choose the Aztec code, because he is capable to contain all the significant data. They prefer the direct method. Maybe security reasons.