Mass. Gen. Laws ch. 58 § 10C
On or before January thirty-first of the year next following the year for which an equalization is to be established, the commissioner shall, on the basis of the equalized valuations determined under sections nine and ten A, as modified by the appellate tax board under section ten B, establish a final equalization and apportionment upon the several cities and towns as of January first of such year of the fair cash value of all property in the commonwealth subject to local taxation, the fair cash value of all such property in each city and town, the ratio which the fair cash value of all such property in each city or town bears to the fair cash value of all such property in the commonwealth, and the proportion of county tax, which should be assessed upon each city and town. Prior to establishing a final equalization and apportionment, the commissioner shall add to the equalized valuations so determined the fair cash value of all property subject to the excise imposed by section ten of chapter one hundred and twenty-one A and said property shall, for the purpose of this section only, be deemed property subject to local taxation. The fair cash value of said property shall be established by the commissioner at an amount no higher than that which would result if the owner thereof were paying a tax under the provisions of chapter fifty-nine equal to the total payments made to the city or town under chapter one hundred and twenty-one A in the most recent calendar year, and if all property actually subject to local taxation in such city or town were being assessed at the equalized valuation determined by the commissioner hereunder. The equalized valuations and apportionments so established shall be the basis for all state, county or district assessments, distributions and appropriated grants and reimbursements which are based on equalized valuations and apportionments, in the next two-year period and until another equalization and apportionment has been established.
Mass. Gen. Laws ch. 58, § 10C