Not later than 15 days prior to the scheduled date of the hearing except for good cause shown, or prior to such earlier date as the presiding officer may order, any party may serve upon an opposing party a written request for the admission of the genuineness and authenticity of any relevant documents described in and exhibited with the request, or for the admission of the truth of any relevant matters of fact stated in the request. Each of the matters of which an admission is requested shall be deemed admitted, unless within a period designated in the request (not less than 10 days after service thereof, or within such further time as the presiding officer or the reviewing authority if no presiding officer has yet been designated may allow upon motion and notice) the party to whom the request is directed serves upon the requesting party a sworn statement either denying specifically the matters of which an admission is requested or setting forth in detail the reasons why he cannot truthfully either admit or deny such matters. Copies of requests for admission and answers thereto shall be served on all parties. Any admission made by a party to such request is only for the purposes of the pending proceeding, or any proceeding or action instituted for the enforcement of any order entered therein, and shall not constitute and admission by him for any other purpose or be used against him in any other proceeding or action.