The revision of a decision on which an action is based is subject to the following regulations:

(a) Clear and unmistakable error, § 3.105(a);

(b) Difference of opinion, § 3.105(b);

(c) Character of discharge, § 3.105(c);

(d) Severance of service-connection, § 3.105(d);

(e) Reduction to less than compensable evaluation, § 3.105(e); (See §§ 21.48, 21.322, and 21.324)

(f) Review of decisions, § 21.416.