Originally Posted by
emcampbe
DOT rule as mentioned above, but there are some situations that don't require it. These rules apply to all flights on US carriers and all flights to/from the US only, so doesn't apply everywhere (though other countries potentially could have similar regs). DOT rules don't require the hold or full refund for flights booked within 7 days of travel. United's policy, though, doesn't have this close in restriction.
Minor correction: The DOT
24-hour reservation requirement applies to all airlines (domestic and foreign) who market their flights to consumers in the U.S., regardless of routing. If you'd like more details, see the guidance document linked above.