41 C.F.R. § 301-2.3

(a) Specific authorization or prior approval is required for:

(1) Use of reduced fares for group or charter arrangements;

(2) Use of a foreign air carrier or foreign ship;

(3) Payment of a reduced per diem;

(4) Use of cash to pay for common carrier transportation;

(5) Travel expenses related to emergency travel;

(6) Acceptance of payment from a non-Federal source for travel expenses (see chapter 304 of this subtitle);

(7) Travel expenses related to conference attendance;

(8) Use of a Government aircraft;

(9) Use of extra-fare train service;

(10) Travel by ship; and

(11) Use of a rental car.

(b) Paragraphs (a)(1), (3), (6), and (7) of this section require a written or electronic advance authorization.