While a confirmed ticket in United Economy typically implies you will have a seat on the plane, it is technically not a "guarantee" of a specific seat or even a seat on that specific flight due to the airline's Contract of Carriage. United, like most major carriers, sometimes overbooks flights to account for "no-shows." If you have a "Basic Economy" ticket, you are not assigned a seat until check-in or even at the gate, which means you are at the highest risk of being "bumped" if the flight is oversold. If you have a standard "Economy" ticket and have already selected a seat number, your position is much more secure. However, operational changes—such as an aircraft swap to a smaller plane—can result in seat reassignments. Under U.S. Department of Transportation rules, if you are "involuntarily bumped" from a flight, United is required to compensate you, often with a check for up to 400% of your one-way fare, provided they cannot get you to your destination within a certain timeframe. To best "guarantee" your spot, it is highly recommended to check in exactly 24 hours before departure and join the United MileagePlus program.