The circumstances of the one I got from BRussels to ORD, was that the flight was overbooked in Coach, but there were seats in FC and BC. Therefore, Elite customers in BC were moved to FC, and Fortunate Elite members are moved up to BC, therefore freeing up seats in coach for non-elites who have tickets. They do this before offering lots of vouchers asking people to give up their seat. - So it saves the airline money, and makes some people VERY happy.

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DtG