CRS-3 was the first flight to carry landing legs. The first stage, having delivered Dragon, relit its engines and descended to a controlled touchdown on the ocean surface — not a landing on anything solid, but a demonstration that a booster could be brought back under control rather than simply falling.
The video was largely destroyed by the sea state, but the telemetry showed the stage had reached the surface at survivable velocity in a vertical attitude. It was the step that made land and droneship landings credible.
The cargo included the first laser communications experiment mounted on the station's exterior, a plant growth facility that has since produced several crops of edible vegetables in orbit, and a set of commercial high-definition cameras streaming continuous views of the Earth.
It also carried a pair of legs for the station's robotic assistant and several hundred kilograms of scientific hardware, and returned nearly two tonnes of cargo — the return leg being the capability nothing else in service could provide.
The flight sits at the point where commercial resupply stopped being an experiment. From here the cadence became routine, and the interesting engineering shifted to what happened to the rocket after the payload was delivered.
