Mont. Code § 81-23-104
The board may adopt and enforce rules and orders necessary to carry out the provisions of this chapter and any orders adopted under it by the board. A rule or order must be posted for public inspection in the main office of the department for 30 days, and a copy must be filed in the office of the department. A copy must also be sent by registered or certified letter to the secretary of each area, except in the case of an order directed only to a person or persons named in it, which must be served by personal delivery of a copy or by mailing a copy to each person to whom the order is directed or, in the case of a corporation, to any officer or agent of the corporation upon whom a summons may be served in accordance with laws of this state. The posting, in the main office of the department, of a rule or order not required to be personally served as provided in this section and the filing in the office of the department is sufficient notice to all persons affected by the rule or order. A rule or order when properly posted and filed or served, as provided in this section, has the force of law.
§ 81-23-104, MCA