Mass. Gen. Laws ch. 59 § 25
The assessors of each city or town shall raise by taxation each year a reasonable amount of overlay as the commissioner may approve. The overlay account may be used only for avoiding fractional divisions of the amount to be assessed, for abatements granted on account of property assessed for any fiscal year and for any interest payable on such abatements under section 64 or 69. Any balance in the overlay account in excess of the amount of the warrants remaining to be collected or abated, as certified by the board of assessors, shall be transferred by the board of assessors upon their own initiative or within 10 days of a written request by the chief executive officer, with written notice to the chief executive officer, to a reserve fund to be appropriated for any lawful purpose. Any balance in a reserve fund at the end of the fiscal year shall be closed out to surplus revenue. This section shall apply to fire, water and improvement districts.
Mass. Gen. Laws ch. 59, § 25