United Airlines (UAL) “deplaned” one of its paying passengers late Sunday, dragging him from his seat and down the aisle.

Airlines routinely overbook planes and pay people to take the next flight when necessary. On this particular flight, United needed four seats for employees hitching a ride. It offered $800 vouchers, but no one on the plane volunteered. Instead, United selected four passengers at random.

When one passenger didn’t comply, United had three men forcibly remove him. And soon after, a video of the incident went viral—giving United another round of bad press.

Despite the outrage expressed on social media, and by a concerned passenger in the video, United’s actions indicated it considered itself in the right. The reason is simple: a half-pound stack of paper called the “contract of carriage.”

Like the terms of service agreements most people scroll through quickly and click “I agree,” the United contract of carriage is something you likely agreed to without reading or understanding, something that JPMorgan CEO Jamie Dimon recently said can be trouble.

Airlines don’t value economy customers as much

Under Rule 25—on page 35 if you print it out—the agreement says exactly what happens if the flight is oversold. “If there are not enough volunteers, other Passengers may be denied boarding involuntarily,” the language reads. (Of course, the deplaned man was not denied boarding, he was already boarded.)

The language continues however, shining light on how these “other Passengers” are chosen. It’s not random, it’s “in accordance with UA’s boarding priority.” That means that if you have a certain type of fare class—you may not even know these exist—a complex itinerary, status (e.g. gold or platinum), have checked in early, or have frequent flier card, you are less likely to be asked to take the next flight. Even if it’s just a free frequent flier card that you never use, it might save you from being asked to leave.

For passengers looking to take advantage of the budget seats offered, this unspoken ranking and largely unknown class system is important to know. Though companies take great pains to say otherwise, if you paid less, you are not as valued a customer.

Update 3:30p: The post’s headline has been updated.

Ethan Wolff-Mann is a writer at Yahoo Finance focusing on consumer issues, tech, and personal finance. Follow him on Twitter @ewolffmann. Got a tip? Send it to tips@yahoo-inc.com.

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