Nothing! The sleeve is interchangeable as long as the size is right. You can wear the FREEZE SLEEVE on your elbow, your forearm, your calf, your bicep, your quad, your wrist, your ankle and your knee if the fit is right so be sure and check your measurements! (To size an area like your calf, measure the diameter of the widest part.)
Most people will not use the same size for all body parts. You will need a larger size for your knee than you need for your elbow. Make sure you have the right sizes for the areas you are applying compression cold therapy to.