Although their origin varies between different incarnations of the Legion Of Super-Heroes, the Legion flight rings are usually invented in a joint effort by Brainiac 5 and Invisible Kid. Engraved with the signature "L" of the Legion of Super-Heroes, this ring is also used by students in the Legion Academy when they are in training or on field missions. Legion Flight Rings are composed of a compound known as Valorium, which is derived from the Thanagarian Nth Metal, and possesses similar anti-gravity capabilities.
A Legion Flight Ring is controlled by mental command, and like a Green Power Ring, it can be willed for varying effects. For example, because of Dream Girl's strong will, she has used the Flight Ring to move other objects be negating the gravity around it. It's main function, however, is to allow it's wearer to fly.
Another function of the Legion Flight Ring is to send out a distress call. If a Legionnaire rotates the "L" symbol 45 degrees, the ring will send out a distress message that will be recieved by other Legionnaires and the Mission Monitor Board at Legion Headquarters.
Brainiac 5 has modified the functions of a ring specific to each Legionnaire. For example, Colossal Boy's ring will enlarge with him, and Shrinking Violet's will shrink with her. Other examples of this include the Legion Flight Ring being able to "Duplicate" with Duplicate Damsel, resist temperature extremes around Sun Boy and Polar Boy and handle the increased mass of Blok. Another modification he had recently made, was to Earth-Man's Legion Flight Ring, in order to keep him on a short leash and alter his morality.
A Legion Flight Ring is also used for security purposes. Only someone with a Legion Flight Ring is allowed to use certain equipment, including the Legion Cruiser.
In certain alternate universe versions of the Legion, it has other uses as well. Environmental protection is an added benefit for the Earth-Prime Legionnaires, for example. The Earth-0 and Earth-247 Legionnaires use Legion Transuits instead.
Booster Gold is one of the few known non-Legionnaires allowed to use a Legion flight ring.
Superman also possesses a flight ring, and used to keep it in the Fortress of Solitude as a reminder of his time spent in the 31st century. He now wears it, but it can be turned invisible upon mental command.
Superboy (Kon-El) has one for his time with the Legion of Earth-247, and Kid Flash II (Bart Allen) may have one as well.
In Earth-Prime continuity, there is a shortage of rings because the metal is rare, and they are expensive to make. Brainiac 5 has to recycle the rings of deceased members such as Dream Girl. This also means that not everyone was equipped with a ring.