Under regulations prescribed by the Secretary of Defense, an officer of an armed force who is assigned to a permanent duty station at a location outside the United States may be paid a civilian clothing allowance in such amount as the Secretary shall determine under regulations if such officer is required to wear civilian clothing all or a substantial portion of the time in the performance of the officer’s official duties. A clothing allowance under this section is in addition to any uniform allowance to which an officer is otherwise entitled under this title.