39 U.S.C. § 1202: Bargaining units

any management official or supervisor; any employee engaged in personnel work in other than a purely nonconfidential clerical capacity; both professional employees and employees who are not professional employees unless a majority of such professional employees vote for inclusion in such unit; or together with other employees, any individual employed as a security guard to enforce against employees and other persons, rules to protect property of the Postal Service or to protect the safety of property, mail, or persons on the premises of the Postal Service; but no labor organization shall be certified as the representative of employees in a bargaining unit of security guards if such organization admits to membership, or is affiliated directly or indirectly with an organization which admits to membership, employees other than guards.