service after December 31, 2020, for the 10-year period beginning on the date such facility is placed in service, or (ii) in the case of a facility that is originally placed in service before January 1, 2021, for the 10-year period beginning on January 1, 2021. (5) Transportation fuel The term transportation fuel means a fuel which is suitable for use as a fuel in a highway vehicle or aircraft. (f) Final guidance Not later than January 1, 2020, the Secretary, in consultation with the Administrator of the Environmental Protection Agency, shall issue final guidance regarding implementation of this section, including calculation of emissions factors for transportation fuel, the table described in subsection (b)(2), and the determination of clean fuel production credits under this section. (g) Special rules (1) Only registered production in the United States taken into account (A) In general No clean fuel production credit shall be determined under subsection (a) with respect to any transportation fuel unless— (i) the taxpayer is registered as a producer of clean fuel under section 4101 at the time of production, and (ii) such fuel is produced in the United States. (B) United States For purposes of this paragraph, the term United States includes any possession of the United States. (2) Production attributable to the taxpayer In the case of a facility in which more than 1 person has an ownership interest, except to the extent provided in regulations prescribed by the Secretary, production from the facility shall be allocated among such persons in proportion to their respective ownership interests in the gross sales from such facility. (3) Related persons Persons shall be treated as related to each other if such persons would be treated as a single employer under the regulations prescribed under section 52(b). In the case of a corporation which is a member of an affiliated group of corporations filing a consolidated return, such corporation shall be treated as selling fuel to an unrelated person if such fuel is sold