(1) It is presumptively appropriate to exclude persons who exercise authority on behalf of the employer over subordinate employees (usually termed "supervisors") from bargaining units containing their rank-and-file subordinates to avoid a potential for conflicts of interest that would otherwise exist in a combined bargaining unit.

(2) It is presumptively appropriate to include persons who exercise authority on behalf of the employer over subordinate employees (usually termed "supervisors") in separate bargaining units for the purposes of collective bargaining.

(3) The presumptions set forth in this section are subject to modification by adjudication.