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

1. (1) Use of reduced fares for group or charter arrangements;
2. (2) Use of a foreign air carrier or foreign ship;
3. (3) Payment of a reduced per diem;
4. (4) Use of cash to pay for common carrier transportation;
5. (5) Travel expenses related to emergency travel;
6. (6) Acceptance of payment from a non-Federal source for travel expenses (see chapter 304 of this subtitle);
7. (7) Travel expenses related to conference attendance;
8. (8) Use of a Government aircraft;
9. (9) Use of extra-fare train service;
10. (10) Travel by ship; and
11. (11) Use of a rental car.