32 C.F.R. § 631.14 – Army policy.

(a) Soldiers, military and/or Department of the Army Civilian (DAC) police performing off-installation operations must be thoroughly familiar with applicable agreements, constraints of the Posse Comitatus Act (18 U.S.C. 1385) in the Continental United States (CONUS) and United States-host nation agreements in areas OCONUS.
(b) Military and/or DAC police assigned to off-installation operations have the sole purpose of enforcing parts, and orders pertaining to persons subject to their jurisdiction.
(c) Military and/or DAC police accompanying civilian law enforcement officers remain directly responsible to, and under the command of, U.S. Army superiors. Military and DAC police may come to the aid of civilian law enforcement officers to prevent the commission of a felony or injury to a civilian law enforcement officer.
(d) Regional Directors of the Army Installation Management Agency (IMA), Commander, Army Materiel Command (AMC), and Commander, Army Test and Evaluation Command (ATEC) may authorize subordinate commanders to establish off-installation operations within the limits imposed by higher authority, the Posse Comitatus Act (18 U.S.C. 1385) in CONUS, and United States-host nation agreements in OCONUS areas—
1. (1) To assist Federal, State, and local law enforcement agencies.
2. (2) In conjunction with military activities.
3. (3) To safeguard the health and welfare of Soldiers.
4. (4) When the type of offenses or the number of Soldiers frequenting an area is large enough to warrant such operations.
(e) The constraints on the authority of Soldiers and/or DAC police to act off-Installation, (Posse Comitatus Act (18 U.S.C. 1385) in CONUS and United States-host nation agreements in OCONUS areas) and the specific scope of off-installation operations will be clearly delineated in all authorizations for off-installation operations. Off-installation operations will be coordinated with the local installation commander through the Staff Judge Advocate (SJA), or higher authority, and appropriate civilian law enforcement agencies.