Here are some of the ways countries use the documents:

Revocation as Punishment

U.S. law allows a citizen’s passport to be revoked for nonpayment of taxes, but the secretary of state can also deny or take away passports for reasons including national security. The latter rationale was at play in the case of Hoda Muthana, who was born in the United States to a former Yemeni diplomat. She was previously recognized as an American citizen and was granted passports in 2005 and 2014. But after she traveled to ISIS-controlled territory in Syria and urged violence against the West, Barack Obama’s administration said that Muthana had never been eligible for American citizenship, because of her father’s diplomatic status. Muthana is now in a displaced-person’s camp in Syria and wants to return to Alabama, but she can’t. Her case is making its way through the judicial system.

Read: This ISIS citizenship case could set a terrifying precedent

Britain is grappling with a similar case—that of Shamima Begum, who also traveled to ISIS-controlled Syria. Begum is also in a camp but can’t return to England, because the British government stripped her of her citizenship. London argued that it could do so on the grounds that Begum was eligible for Bangladeshi nationality, and so would not have been rendered stateless. Dhaka says she has never been a citizen.

Revenue Generation

Revocations of passports and citizenship are relatively rare. What’s not as unusual is the legal sale of passports as a way for a country to raise money. A Western country’s passport is particularly prized because it allows someone who holds one the right to live and work in that state—and grants them visa-free travel across much of the world. Three European countries sell outright citizenship that comes with a passport: Bulgaria’s can be bought for about $600,000; Cyprus’s for about $1.75 million; and Malta’s for about $1 million. (Other countries, especially those in the Caribbean, offer citizenship for a far lower price.) These offers are typically taken up by either superrich people who want to pay lower levels of taxes or by the citizens of countries such as China and Russia who want to live somewhere with rule of law and a better quality of life.

Read: The EU wants to clamp down on the super rich’s visa of choice

Travel Restriction

Authoritarian regimes frequently deny passports to dissidents to prevent them from leaving. Iran, China, and North Korea are examples of this. Other countries, including India, have placed travel restrictions on their lowest-skilled workers because of the possibility of exploitation overseas—though enforcement is lax. Some states with a record of persecution of domestic minorities, such as Myanmar (also known as Burma), make it near impossible for those groups—such as Rohingya Muslims, in the case of Myanmar—to get travel documents.