A Denied Boarding Form is the document that airlines must issue to a passenger who is left on the ground due to overbooking. It is a paper that must clearly state the passenger's name, flight number, date, itinerary, reason being overbooking and the amount of compensation you are entitled to. It is important that you do not accept any offer made at the airport counter such as: vouchers, points, future discounts, etc. Some airlines use this technique to offer a lower compensation than what is legally established. In this way, you will lose the right to claim the correct/higher amount later on.