38 C.F.R. § 4.27
The diagnostic code numbers appearing opposite the listed ratable disabilities are arbitrary numbers for the purpose of showing the basis of the evaluation assigned and for statistical analysis in the Department of Veterans Affairs, and as will be observed, extend from 5000 to a possible 9999. Great care will be exercised in the selection of the applicable code number and in its citation on the rating sheet. No other numbers than these listed or hereafter furnished are to be employed for rating purposes, with an exception as described in this section, as to unlisted conditions. When an unlisted disease, injury, or residual condition is encountered, requiring rating by analogy, the diagnostic code number will be "built-up" as follows: The first 2 digits will be selected from that part of the schedule most closely identifying the part, or system, of the body involved; the last 2 digits will be "99" for all unlisted conditions. This procedure will facilitate a close check of new and unlisted conditions, rated by analogy. In the selection of code numbers, injuries will generally be represented by the number assigned to the residual condition on the basis of which the rating is determined. With diseases, preference is to be given to the number assigned to the disease itself; if the rating is determined on the basis of residual conditions, the number appropriate to the residual condition will be added, preceded by a hyphen. Thus, rheumatoid (atrophic) arthritis rated as ankylosis of the lumbar spine should be coded "5002-5240." In this way, the exact source of each rating can be easily identified. In the citation of disabilities on rating sheets, the diagnostic terminology will be that of the medical examiner, with no attempt to translate the terms into schedule nomenclature. Residuals of diseases or therapeutic procedures will not be cited without reference to the basic disease.
38 C.F.R. §4.27