14 CFR § 21.25 - Issue of type certificate: Restricted category aircraft.
(a) An applicant is entitled to a type certificate for an aircraft in the restricted category for special purpose operations if he shows compliance with the applicable noise requirements of Part 36 of this chapter, and if he shows that no feature or characteristic of the aircraft makes it unsafe when it is operated under the limitations prescribed for its intended use, and that the aircraft -
(2) Is of a type that has been manufactured in accordance with the requirements of and accepted for use by, an Armed Force of the United States and has been later modified for a special purpose.
(b) For the purposes of this section, “special purpose operations” includes -
(1) Agricultural (spraying, dusting, and seeding, and livestock and predatory animal control);
(2) Forest and wildlife conservation;
(3) Aerial surveying (photography, mapping, and oil and mineral exploration);
(4) Patrolling (pipelines, power lines, and canals);
(5) Weather control (cloud seeding);
(6) Aerial advertising (skywriting, banner towing, airborne signs and public address systems); and
(7) Any other operation specified by the FAA.