Mass. Gen. Laws ch. 147 § 13A
A town having an organized police force or department, other than a town in which a reserve police force is established under special law, which accepts the provisions of this section, may establish a reserve police force consisting of such number of members as the town may determine. Appointments to such force shall be made in the same manner and subject to the same provisions of law as appointments to its regular police force. Members of said reserve force may be removed by the selectmen at any time for any reason satisfactory to them and shall be subject to such rules and regulations as the selectmen may prescribe. Said members shall, when on duty, have all the powers and duties of members of the regular police force of said town and shall be paid by the town such compensation as the selectmen may fix.
Mass. Gen. Laws ch. 147, § 13A