SAN FRANCISCO, CA - JUNE 10: A United Airlines plane sits on the tarmac at San Francisco International Airport on June 10, 2015 in San Francisco, California. The Environmental Protection Agency is taking the first steps to start the process of regulating greenhouse gas emissions from airplane exhaust. (Photo by Justin Sullivan/Getty Images)
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April 11, 2017 5:09 PM EDT

The United Airlines flight that a passenger was forcibly removed from after refusing to vacate his seat was not overbooked, as the airline originally stated, but those on board were still asked to step aside for “must-ride” passengers — employees who needed to be in Kentucky the following day, United spokesman Jonathan Guerin told USA Today.

But what are these “must-ride” passengers, and why do they take priority over other people on board?

Simply put, a must ride passenger is an airline employee who gets seating priority on a flight because they are needed in a different location. In a USA Today article from December 2014, retired airline pilot John Cox was asked by one flier why she had been waiting at the gate for an hour for one of these “must ride” passengers. “What does that mean?” the passenger asked. “There are so many flights on this route (Phoenix to Los Angeles), why are we waiting an hour?”

A “must ride” is a company employee that is needed in another city,” Cox wrote. “Examples could be a pilot or flight attendant needed to crew an airplane in another city that has had the planned crew get ill, or a maintenance technician needed to repair an airplane that has experienced a mechanical issue.”

Ultimately, Cox concluded, putting a must-ride passenger on a plane is a “judgment decision made by highly experienced people in the operations center.”

Write to Alana Abramson at Alana.Abramson@time.com.

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