5 U.S.C. § 5925
Post differentials
(a)
A post differential may be granted on the basis of conditions of environment which differ substantially from conditions of environment in the continental United States and warrant additional pay as a recruitment and retention incentive. A post differential may be granted to an employee officially stationed in the United States who is on extended detail in a foreign area. A post differential under this subsection may not exceed 35 percent of the rate of basic pay.
(b)
Any employee granted a differential under subsection (a) of this section may be granted an additional differential for an assignment to a post determined to have especially adverse conditions of environment which warrant additional pay as a recruitment and retention incentive for the filling of positions at that post. An additional differential for any employee under this subsection—
(1)
may be paid for each assignment to a post determined to have such conditions;
(2)
may be paid periodically or in a lump sum; and
(3)
may not exceed 15 percent of the rate of basic pay of that employee for the period served under that assignment.
( Pub. L. 89–554 , Sept. 6, 1966 , 80 Stat. 512 ; Pub. L. 96–465, title II, § 2309 , Oct. 17, 1980 , 94 Stat. 2165 ; Pub. L. 108–199, div. D, title V, § 591(a) , Jan. 23, 2004 , 118 Stat. 207 ; Pub. L. 109–140, § 4(b) , Dec. 22, 2005 , 119 Stat. 2651 .)