https://www.law.cornell.edu/cfr/text/5/213.3102

(i) Temporary and less-than-full time positions for which examining is impracticable. These are:

(1) Positions in remote/isolated locations where examination is impracticable. A remote/isolated location is outside the local commuting area of a population center from which an employee can reasonably be expected to travel on short notice under adverse weather and/or road conditions which are normal for the area. For this purpose, a population center is a town with housing, schools, health care, stores and other businesses in which the servicing examining office can schedule tests and/or reasonably expect to attract applicants. An individual appointed under this authority may not be employed in the same agency under a combination of this and any other appointment to positions involving related duties and requiring the same qualifications for more than 1,040 workings hour in a service year. Temporary appointments under this authority may be extended in 1-year increments, with no limit on the number of such extensions, as an exception to the service limits in § 213.104.

(2) Positions for which a critical hiring need exists. This includes both short-term positions and continuing positions that an agency must fill on an interim basis pending completion of competitive examining, clearances, or other procedures required for a longer appointment. Appointments under this authority may not exceed 30 days and may be extended for up to an additional 30 days if continued employment is essential to the agency‘s operations. The appointments may not be used to extend the service limit of any other appointing authority. An agency may not employ the same individual under this authority for more than 60 days in any 12-month period.

(3) Other positions for which OPM determines that examining is impracticable.