A cardinal rule of appeals it that the record on appeal contains only documents that were before the trial court, and counsel may not refer to matters outside, or "dehors," the record (see Citibank, N.A. v Kerszko, 203 AD3d 42, 53 [2d Dept. 2022]). After all, the appeal is about reviewing what already transpired, not redoing the trial proceedings. Therefore, practitioners often successfully move to strike offending matters from outside the record that are improperly included in records, briefs or appendices (see e.g. West 125th St. Realty LLC v Chosen Realty Corp., 227 AD3d 431, 432 [1st Dept 2024]).

Further, attorneys who report new developments at oral argument may be rebuked. However, there are exceptions to the general rule. 

Matters Outside the Record

An appellate court may take judicial notice of certain facts (see Matter of Albany v. Kirby, 36 NY2d 526, 532 [1975]) and of laws specified in CPLR 4511, and may consider certain incontrovertible documentary evidence (see Crawford v. Merrill Lynch, Pierce, Fenner & Smith, 35 NY2d 291, 298-300 [1974]). Another exception has been carved out for cases involving the best interests of children in custody proceedings. First enunciated in Matter of Michael B., 80 NY2d 299, 317-318 (1992), this narrow exception has been applied in hundreds of cases since then. Yet case law is somewhat opaque about how relevant matters should be presented to the appellate court.