The term party, whenever used in these Rules, shall include any natural person, corporation, association, firm, partnership, trustee, receiver, agency, public or private organization, or governmental agency. A party requesting official action subject to these Rules shall be designated as applicant. A party whose petition for leave to intervene is granted pursuant to Sec. 201.78 shall be designated as intervenor. Only a party as designated in this section may introduce evidence or examine witnesses at hearings.