respect to any beneficiary for any calendar year, the lesser of— (i) the excess (if any) of— (I) the zero bracket threshold, over (II) the sum of— (aa) the net value of all taxable assets of the beneficiary for the calendar year, plus (bb) any unused 0 percent bracket amount assigned by the beneficiary to other nongrantor multibeneficiary trusts, or (ii) the portion of the net value of all taxable assets of the trust which such beneficiary is eligible to receive. (C) Unused 2 percent bracket amount For purposes of this paragraph, the term unused 2 percent bracket amount means, with respect to any beneficiary for any calendar year, the lesser of— (i) the excess (if any) of— (I) the top bracket threshold reduced by the zero bracket threshold, over (II) the sum of— (aa) the net value of all taxable assets of the beneficiary for the calendar year in excess of the zero bracket threshold, plus (bb) any unused 2 percent bracket amount assigned by the beneficiary to other nongrantor multibeneficiary trusts, or (ii) the portion of the net value of all taxable assets of the trust which such beneficiary is eligible to receive. (D) Assignment of amounts The assignment of any amount of unused 0 percent bracket amount and unused 2 percent bracket amount shall be made at such time and in such manner as specified by the Secretary in regulations. In any case in which no affirmative assignment is made by a beneficiary, the amount assigned shall be $0. (3) Nongrantor multibeneficiary trust For purposes of this chapter— (A) In general The term nongrantor multibeneficiary trust means any trust or portion of a trust— (i) with respect to which no person is treated as an owner under subpart E of subchapter J of chapter 1, (ii) no property of which is attributable to a gratuitous transfer of assets by a person who is subject to tax under this chapter for the calendar year, and (iii) which has more than one beneficiary (determined as of the last day of the calendar year). (B) Exception Such term shall not include— (i) any trust described