5 CFR §575.402

1. (a)The head of an agency may pay a supervisory differential to a supervisor who is—
   1. (1)In a General Schedule position paid under 5 U.S.C. 5332; and
   2. (2)Responsible for providing direct, technical supervision over the work of one or more civilian employees whose positions are not under the General Schedule if the continuing pay (as determined under § 575.405(d) of this part) of one or more of the subordinates would, in the absence of such a differential, be more than the continuing pay (as determined under § 575.405(c) of this part) of the supervisor.
2. (b)A supervisory differential may not be paid on the basis of supervising a civilian employee whose rate of basic pay exceeds the maximum rate of basic pay established for grade GS-15 on the pay schedule applicable to the GS supervisor, including a schedule for any applicable special rate under 5 CFR part 530, subpart C; locality-based comparability payment under 5 CFR part 531, subpart F; or similar payment or supplement under other legal authority.