Well, first, it is an objective finding of medical impairment to a particular body part that is used world-wide to document impairment. It is one of the biggest "value driver" on a California Personal Injury Case; and it is the best arsenal an experienced injury lawyer can use to set a foundation for future medical care pre-trial and during a jury trial.
In California, we use the American Medical Association (AMA) Guidelines, 5th Edition, and it usually done, when warranted and supported by the medical findings, usually after the course and care of therapy is completed by the client/patient. If done properly, and correctly it objectifies an injury case to show how and why a body part does not work as it did before. It is so good that the California Labor Code adopted it around 2005 to begin documenting impairments to injured workers throughout California. Your personal injury needs to not only know about it, but how to argue for it on your California San Diego Personal Injury case. The following is a short video I produced to help explain the AMA Guidelines, and their important your accident case: