37 CFR 41.69: Examiner's answer.

(a) The primary examiner may, within such time as directed by the Director, furnish a written answer to the owner's and/or requester's appellant brief or respondent brief including, as may be necessary, such explanation of the invention claimed and of the references relied upon, the grounds of rejection, and the reasons for patentability, including grounds for not adopting any proposed rejection. A copy of the answer shall be supplied to the owner and all requesters. If the primary examiner determines that the appeal does not comply with the provisions of §§ 41.61, 41.66, 41.67 and 41.68 or does not relate to an appealable action, the primary examiner shall make such determination of record.

(b) An examiner's answer may not include a new ground of rejection.

(c) An examiner's answer may not include a new determination not to make a proposed rejection of a claim.

(d) Any new ground of rejection, or any new determination not to make a proposed rejection, must be made in an Office action reopening prosecution.