Subject to the limitations provided under s. 71.26 (3) (n), for each taxable year that a corporation that is a member of a combined group has an unused credit or credit carry-forward under s. 71.28 (4) or (5) or 71.47 (4) or (5), the corporation may, after using that credit or credit carry-forward to offset its own tax liability for the taxable year, use that credit or credit carry-forward to offset the tax liability of all other members of the combined group on a proportionate basis, to the extent such tax liability is attributable to the unitary business.

Unless otherwise provided by the department by rule, if the corporation may no longer be included in the combined group, as determined by this section, the corporation’s unused credits shall be available only to that corporation.