39 U.S.C. § 1201

As used in this chapter, “guards” means— (1) maintenance guards who, on the effective date of this chapter, are in key position KP–5 under the provisions of former section 3514 of title 39 ; and (2) security guards, who may be employed in the Postal Service and whose primary duties shall include the exercise of authority to enforce rules to protect the safety of property, mail, or persons on the premises.