The commissioner shall determine the part of the net income of a financial institution derived from business carried on within the commonwealth as follows:

1. (a) If the financial institution does not have income from business activity which is taxable in another state, the whole of its net income shall be taxable under section two. For purposes of this section, a financial institution is taxable in another state as defined in the definition of ''taxable'' in section one. Notwithstanding any other provision of this section, the portion of the net income of a financial institution that a nondomiciliary state is prohibited from taxing under the Constitution of the United States shall be allocated in full to the commonwealth if the commercial domicile of the institution is in the commonwealth.

   [Subsection (b) effective for taxable years beginning on or after January 1, 2025. See 2023, 50, Sec. 47 as amended by 2023, 77, Sec. 194.]