Within 2 months of the filing of an appeal brief, the Examiner furnishes the appellant a written statement in answer to the appellant's brief. The statement is called the Examiner's answer. The appellant then may file a reply brief directed only to new points of argument raised in the Examiner's answer. The reply brief must be filed within 2 months of the date of mailing of the Examiner's answer to avoid dismissal of the appeal as to claims subject to any new ground of rejection raised in the Examiner's answer. This response period is one of the few that may not be extended by petition and payment of a fee under 37 CFR 1.136(a).
The position of the USPTO on this issue is described in the following section(s) of the Manual of Patent Examining Procedure (MPEP):
MPEP 1208.03 - Reply Brief
Law - Reply Brief