Probate Code § 16341

A fiduciary shall allocate to income an amount received as a distribution of income, including a unitrust distribution under Article 3 (commencing with Section 16330), from a trust or estate in which the fiduciary has an interest, other than an interest the fiduciary purchased in a trust that is an investment entity, and shall allocate to principal an amount received as a distribution of principal from the trust or estate. If a fiduciary purchases, or receives from a settlor, an interest in a trust that is an investment entity, Section 16340, 16354, or 16355 applies to a receipt from the trust.