Stipulations. The parties, by stipulation, may agree to any substantive or procedural matter. A stipulation may be filed in writing or entered on the record at the hearing. Affidavits. A party, the Board or any Board staff, or the presiding hearing officer, because a witness is unavailable for another appropriate reason, may admit an affidavit as evidence. All affidavit testimony shall be under oath or affirmation and subject to the penalties of perjury.