52 Pa. Code § 1001.57
(a) One copy of a document shall be served on the presiding officer if one has been designated. The following number of copies of documents shall be served on other parties in a proceeding:
(1) Briefs.
This section cited in 52 Pa. Code § 1001.52 (relating to service by a party); 52 Pa. Code § 1005.192 (relating to filing and service of briefs); and 52 Pa. Code § 1011.4 (relating to annual assessments and renewal fees).