[Enterprise]

For simplifications sake, let's suppose the plane has a capacity for 100 passengers. They board the plane, and all the seats are filled. The plane is not "overbooked," it is full.

A situation came up where United needed to get 4 people to Louisville. So 4 people needed to give up their seats. 3 people volunteered, but it wasn't quite enough. And the rest is history.

[FreedomPoster]

They didn’t volunteer, they simply didn’t object when told to leave, is my understanding.