41 CFR § 301-2.5 - What travel arrangements require specific authorization or prior approval?
You must have a specific authorization or prior approval for:
(a) Use of other than coach-class service on common carrier transportation;
(b) Use of a foreign air carrier;
(c) Use of reduced fares for group or charter arrangements;
(d) Use of cash to pay for common carrier transportation;
(e) Use of extra-fare train service;
(f) Travel by ship;
(g) Use of a rental car;
(h) Use of a Government aircraft;
(i) Payment of a reduced per diem rate;
(j) Payment of actual expense, unless your agency has issued a blanket actual expense authorization under § 301-70.201;
(k) Travel expenses related to emergency travel;
(l) Transportation expenses related to threatened law enforcement/investigative employees and members of their families;
(m) Travel expenses related to travel to a foreign area;
(n) Acceptance of payment from a non-Federal source for travel expenses, see chapter 304 of this subtitle;
(o) Travel expenses related to attendance at a conference; and
(p) Due to an employee's medical requirements or religious beliefs, payment of the full M&IE allowance even though meals are furnished by the Government either directly or through a registration fee or other payment for a conference or other event, in accordance with § 301-11.18(b).
Paragraphs (c), (i), (n), and (o) of this section require a written or electronic advance authorization.