Swivel Clip: A swivel clip prevents your leash from twisting into a knot. This would make the retractable functionality stop working altogether.
Locking Type/Mechanism: Most retractable leashes offer a stop or lock button. These press against the coil of leash available, or against the strand currently being released. This stops the leash from being released any further.
Cord or Tape Leash Type: Cord-style leashes are often the lighter-weight option and can work for smaller dogs. However, they also pose a greater risk of possible injury. Separately, the tape-style leash is stronger and can work for small and large dogs.
Attachments: These make retractable leashes even more useful. Some of the most popular attachments are; an attachable flashlight, poop bag holder, and even a treat holding pouch.