The error must be based exclusively on the record and the statutory law that existed when the prior decision was rendered. You cannot introduce new medical evidence created after the decision date.
The adjudicator must have committed an error of fact or law (e.g. failed to apply a mandatory presumption, failed to apply bilateral factor, or misread clear STR medical entries).
The error must be so critical that, had it not occurred, the outcome would have been manifestly different (e.g. service connection would have been granted instead of denied).