(a) If the commissioner orders the detachment or annexation of a portion of a parcel or item of property under this subchapter, the order shall specify the portion of the taxable value of the property to be detached or annexed and may, but need not, describe the specific area of the parcel or item to be detached or annexed.
(b) If an order for the detachment or annexation of a portion of a parcel or item of property does not describe the specific area of the parcel or item to be detached or annexed, the commissioner, as soon as practicable after issuing the order, shall determine the specific area to be detached or annexed and shall certify that determination to the appraisal district for the county in which the property is located.

(c) If portions of a parcel or item of property are located in two or more school districts as the result of a detachment or annexation, the parcel or item shall be appraised for taxation as a unit, and the commissioner shall determine the portion of the taxable value of the property that is located in each of those school districts based on the square footage of the property, or any other reasonable method adopted by the commissioner.