That would be the Wind SR Sword in the exchange, for the world 2 items iirc. What it means is that you can buy each copy ONCE (they're identical). Ideally, of course, you'd use it for Break Limiting the same copy. The wind sword is the only one kinda maybe worth doing - the other 2 (water staff and thunder lance iirc) are both defender items.

So if you spend a long time farming the wind drop items from the main quests, you could trade them for multiple copies of the SR sword, same as the SR Eidolon Sephirot.