5 CFR §591.230

1. (a)OPM establishes post differential areas in response to agency requests when—
   1. (1)Conditions of environment within the post differential area differ substantially from conditions of environment in the continental United States, and
   2. (2)The major Federal employers within the area believe payment of a post differential is warranted as a recruitment incentive to attract candidates from outside the post differential area to work for the Government in the post differential area.
2. (b)If a department or agency wants OPM to consider establishing or revising the definition of a post differential area, the head of the department or agency or his or her designee must submit a request in writing to OPM.