22 U.S. Code § 3949 - Limited appointments
A limited appointment in the Service, including an appointment of an individual who is an employee of an agency, may not exceed 5 years in duration and, except as provided in subsection (b), may not be extended or renewed. A limited appointment in the Service which is limited by its terms to a period of one year or less is a temporary appointment.
(b) A limited appointment may be extended for continued service—
in accordance with section 3951(a) of this title;
as a career candidate, if continued service is determined appropriate to remedy a matter that would be cognizable as a grievance under subchapter XI;
as a career employee in another Federal personnel system serving in a Foreign Service position on detail from another agency; and