What does it mean to be “bumped?”

The mission needs of space-required passengers or cargo may require the removal of Space-A passengers at any point. If removed after being manifested (approved for a particular flight) on a flight or en route, you may re-register with the date and time adjusted to reflect the date and time of registration at the point of origin. The Space-A passengers will be placed no higher than the bottom of the category I on the Space-A register. Space-A passengers cannot be bumped by other Space-A passengers.