A licensee must appear in person at the Department to answer a notice of hearing or a subpoena duces tecum served upon that licensee. If the licensee is an individual, he or she must appear; if a partnership, one of its general partners must appear; and if a corporation, one of its officers must appear. A notice of hearing or subpoena duces tecum may be served by ordinary mail addressed to the licensee's place of business. They may also be served by ordinary mail addressed to the residence of an individual licensee; the residence of a general partner of a partnership licensee; or the residence of an officer or principal stockholder of a corporate licensee.