38 CFR 20.1103: Rule 1103. Finality of determinations of the agency of original jurisdiction where issue is not appealed.

A determination on a claim by the agency of original jurisdiction of which the claimant is properly notified is final if an appeal is not perfected as prescribed in § 19.52 of this chapter. If no Notice of Disagreement is filed as prescribed in subpart C of this part, the claim shall not thereafter be readjudicated or allowed, except as provided by 38 U.S.C. 5104B or 5108, or by regulation.