Mass. Gen. Laws ch. 203D, § 11

A trustee shall allocate to income an amount received as a distribution of income from a trust or an estate in which the trust has an interest other than a purchased interest, and shall allocate to principal an amount received as a distribution of principal from such a trust or estate. If a trustee purchases an interest in a trust that is an investment entity, or a decedent or donor transfers an interest in the trust to a trustee, section 10 or 24 applies to a receipt from the trust.