An impairment rating is a rating that tries to assign a value to your best recovered state after an injury. Lets say that you had normal grip strength and now you have 75% of that grip strength. There are tests and charts and exams etc.. that can determine what percentage this would be. Every state can be different because it involves workers comp too. So the orthopedist may determine that you have a 15% impairment rating to your hand as a whole and 3% to the arm as a whole. Then you would have to see how much that translates to under your work comp laws.