26 C.F.R. § 1.643(b)-1
For purposes of subparts A through D, part I, subchapter J, chapter 1 of the Internal Revenue Code, "income," when not preceded by the words "taxable," "distributable net," "undistributed net," or "gross," means the amount of income of an estate or trust for the taxable year determined under the terms of the governing instrument and applicable local law. Trust provisions that depart fundamentally from traditional principles of income and principal will generally not be recognized. For example, if a trust instrument directs that all the trust income shall be paid to the income beneficiary but defines ordinary dividends and interest as principal, the trust will not be considered one that under its governing instrument is required to distribute all its income currently for purposes of section 642(b) (relating to the personal exemption) and section 651 (relating to simple trusts). Thus, items such as dividends, interest, and rents are generally allocated to income and proceeds from the sale or exchange of trust assets are generally allocated to principal. However, an allocation of amounts between income and principal pursuant to applicable local law will be respected if local law provides for a reasonable apportionment between the income and remainder beneficiaries of the total return of the trust for the year, including ordinary and tax-exempt income, capital gains, and appreciation. For example, a state statute providing that income is a unitrust amount of no less than 3% and no more than 5% of the fair market value of the trust assets, whether determined annually or averaged on a multiple year basis, is a reasonable apportionment of the total return of the trust. Similarly, a state statute that permits the trustee to make adjustments between income and principal to fulfill the trustee's duty of impartiality between the income and remainder beneficiaries is generally a reasonable apportionment of the total return of the trust. Generally, these adjustments are permitted by state statutes when the trustee invests and manages the trust assets under the state's prudent investor standard, the trust describes the amount that may or must be distributed to a beneficiary by referring to the trust's income, and the trustee after applying the state statutory rules regarding the allocation of receipts and disbursements to income and principal, is unable to administer the trust impartially. Allocations pursuant to methods prescribed by such state statutes for apportioning the total return of a trust between income and principal will be respected regardless of whether the trust provides that the income must be distributed to one or more beneficiaries or may be accumulated in whole or in part, and regardless of which alternate permitted method is actually used, provided the trust complies with all requirements of the state statute for switching methods. A switch between methods of determining trust income authorized by state statute will not constitute a recognition event for purposes of section 1001 and will not result in a taxable gift from the trust's grantor or any of the trust's beneficiaries. A switch to a method not specifically authorized by state statute, but valid under state law (including a switch via judicial decision or a binding non-judicial settlement) may constitute a recognition event to the trust or its beneficiaries for purposes of section 1001 and may result in taxable gifts from the trust's grantor and beneficiaries, based on the relevant facts and circumstances. In addition, an allocation to income of all or a part of the gains from the sale or exchange of trust assets will generally be respected if the allocation is made either pursuant to the terms of the governing instrument and applicable local law, or pursuant to a reasonable and impartial exercise of a discretionary power granted to the fiduciary by applicable local law or by the governing instrument, if not prohibited by applicable local law. This section is effective for taxable years of trusts and estates ending after January 2, 2004.
26 C.F.R. §1.643(b)-1