Overbooking is a 'tactics' widely used by carriers, which consists of selling more tickets than there are physically seats on the plane. It often happens that some one at the last minute resigns from the flight, so in such a way the lines can reach a very high occupancy rate of their machines. Unfortunately - when all passengers appear to the check, some of them can not have their places.
Then the airlines usually offer various types of vouchers and other benefits for passengers, in order to tempt them to voluntarily resign from their places and to go on a trip at a later date. This case is treated as a "voluntary denied boarding", which results in the passenger’s loss of right to compensation.
However, if there are no passengers who voluntarily resign from his seat on the plane with an increased number of bookings, airlines will be forced to redirect some of the passengers on the later flight. Such a case qualifies as 'denied boarding against the will of the passenger', which entitles passengers to compensation in the amount of 250 Euro to 600 Euro, according to the same principles as in case of delays or cancellation of flights.