For tagging purposes this tag refers to metal cuffs used for restraint of the arms or legs, typically used in a dungeon or prison type setting when not used as an accessory. They're similar to the cuffs used under the handcuffs tag, but are depicted more solid, wider, and frequently depicted as simply a solid band of metal around the leg or arm. They may be attached to something else, such as a wall or a ball and chain restraint. Oni are typically seen wearing shackles.
For non-metal cuffs used for the same purpose, simply use just the cuffs tag instead. The word shackles carries undertones of being locked in a dungeon or a dark room.