5 C.F.R. § 304.103
(a) Basic authority.
(b) Inappropriate use. An agency must not use 5 U.S.C. 3109 to appoint an expert or consultant:
(c) Reappointment. An agency may reemploy an expert or consultant to perform demonstrably different duties without regard to the length of that individual's previous expert or consultant service with the agency. Reappointment to perform substantially the same duties is subject to the following limits:
(2) An agency may reappoint an expert or consultant who works on a part-time or intermittent schedule in accordance with one of the following options. The agency must determine which option it will use in advance of any reappointment and must base its determination on objective criteria (e.g., nature of duties, pay level, whether or not work is regularly scheduled). Option 1 must be applied to reappointments of experts and consultants appointed without compensation.