Aztec
Aztec is the code with a little target in the middle, the one on a train ticket or on a boarding pass held up on a telephone screen.
What the word means
It is built outward from the centre, in rings, around a pattern of squares that looks like a target. A reader finds that centre first and works its way out from there.
Unlike the others it needs no clear margin around it, which is worth several millimetres on a small ticket. For the same text it takes less room than the square codes it competes with.
What it does on a real document
On a rail ticket it carries the journey, the passenger and the proof that the ticket was really issued. The inspector's reader takes it off paper, or off a screen held up in the aisle.
The engine draws it on the boarding-pass example, black on white, at the exact size asked for. That is what makes it survive both being printed small and being shown on a screen at half brightness.
Where it comes from, and why
It was invented in 1995 in the United States, at a firm called Welch Allyn, and released for anybody to use rather than licensed. It is an international standard under the number ISO/IEC 24778.
It was made for the awkward cases: a lit screen instead of paper, a scratched window between the reader and the code, a ticket with no margin to spare. That is why the railways of Europe settled on it, and why it is on the pass you hold up at an airport gate.