Per smartertravel.com here are more:
If your flight is canceled, substantially delayed, or rescheduled you have the right to reroute at no extra cost or to receive a full refund, even on a nonrefundable ticket. Airline policies do differ so be sure to check what is meant by substantial delay or schedule change.
Tarmac delays are also addressed. The DOT mandates that an airline may not keep you on a plane for more than three hours on a domestic flight or four hours on an international flight without allowing you to get off if you wish. These are subject to security and safety considerations. The airline is obligated to provide food and water after two hours delay, provide updates to the passengers every 30 minutes, and assure lavatories are operable. If an airline violates these rules, you do not receive compensation. The DOT fines the airline.