Mich. Comp. Laws § 423.9e: Bargaining unit.

Sec. 9e.

The commission, after consultation with the parties, shall determine such a bargaining unit as will best secure to the employees their right of collective bargaining. The unit shall be either the employees of 1 employer employed in 1 plant or business enterprise within this state, not holding executive or supervisory positions, or a craft unit, or a plant unit, or a subdivision of any of the foregoing units. If the group of employees involved in the dispute was recognized by the employer or identified by certification, contract, or past practice, as a unit for collective bargaining, the commission may adopt that unit.