With respect to any undertaking subject to section 306108 of this title that adversely affects any historic property for which a Federal agency has not entered into an agreement pursuant to regulations issued by the Council, the head of the agency shall document any decision made pursuant to section 306108 of this title. The head of the agency may not delegate the responsibility to document a decision pursuant to this section. Where an agreement pursuant to regulations issued by the Council has been executed with respect to an undertaking, the agreement shall govern the undertaking and all of its parts.