38 CFR § 4.1 - Essentials of evaluative rating.
This rating schedule is primarily a guide in the evaluation of disability resulting from all types of diseases and injuries encountered as a result of or incident to military service. The percentage ratings represent as far as can practicably be determined the average impairment in earning capacity resulting from such diseases and injuries and their residual conditions in civil occupations. Generally, the degrees of disability specified are considered adequate to compensate for considerable loss of working time from exacerbations or illnesses proportionate to the severity of the several grades of disability. For the application of this schedule, accurate and fully descriptive medical examinations are required, with emphasis upon the limitation of activity imposed by the disabling condition. Over a period of many years, a veteran's disability claim may require reratings in accordance with changes in laws, medical knowledge and his or her physical or mental condition. It is thus essential, both in the examination and in the evaluation of disability, that each disability be viewed in relation to its history.