Under regulations prescribed by the Secretary of Defense, or the Secretary of Homeland Security with respect to the Coast Guard when it is not operating as a service in the Navy, a member who is assigned to a high priority unit of the Selected Reserve of the Ready Reserve of an armed force, as designated under subsection (b), and who performs inactive duty for training for compensation under section 206 of this title with such unit may be paid compensation, in addition to the compensation to which the member is otherwise entitled, in an amount not to exceed $50 for each regular period of instruction, or period of appropriate duty, at which the member is engaged for at least four hours, including any such instruction or duty performed on a Sunday or holiday.

The Secretary concerned may designate a unit, for the purposes of subsection (a) and under such terms and conditions as the Secretary considers appropriate, as a high priority unit if that unit has experienced, or reasonably might be expected to experience, critical personnel shortages. The Secretary may vacate a designation made under this subsection at any time he considers the designation no longer necessary.

Additional compensation may not be paid under this section for inactive duty performed after December 31, 2018.