17 CFR § 12.20 - Response to counterclaim; reply; election of procedure.
(a) Response to counterclaim. If an answer asserts a counterclaim, the complainant shall, within thirty (30) days after service upon him of the answer by the respondent: (1) Satisfy the counterclaim as if it were a complaint, in the manner prescribed by § 12.17 of these rules; or (2) file a reply to the counterclaim with the Commission.
(b) Form and content of reply. Should the complainant, under this paragraph, elect to file a reply to a counterclaim, the reply shall be strictly confined to the matters alleged in the counterclaim and shall conform to the form and content and other requirements set forth in § 12.18 of these rules.
(c) Election of decisional procedure. If neither the complainant nor the respondent, in the complaint or answer respectively, has previously made an election of the summary decisional procedure or the formal decisional procedure, the complainant may make such an election in his reply.