5 U.S.C. § 5925

1. (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.
2. (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. (1)may be paid for each assignment to a post determined to have such conditions;
   2. (2)may be paid periodically or in a lump sum; and
   3. (3)may not exceed 15 percent of the rate of basic pay of that employee for the period served under that assignment.