President Donald Trump signed an executive order Wednesday that will yank federal funding from sanctuary cities. Some of Trump’s most popular campaign promises centered on immigration, particularly his vow to build a wall on the border between the U.S. and Mexico and deport the estimated 11 million people in the country without documentation. His executive order will force cities that identify themselves as a sanctuary for undocumented immigrants to decide if maintaining the status is worth losing federal funding.

What is a sanctuary city?

While the exact policies differ between cities, a sanctuary city is generally a place that doesn’t inquire about the immigration status of its citizens or cooperate with federal immigration authorities like Immigration and Customs Enforcement. People who commit crimes under the jurisdiction of a sanctuary city are fined or serve the appropriate jail time, but even if they are undocumented they are not necessarily turned over to ICE.

Does living in a sanctuary city mean you can’t be deported?

No. Officials in sanctuary cities are still required to cooperate with federal authorities in investigating crimes, which can lead to deportation. Trump said he will focus on deporting criminals who are in the country without papers.

How many sanctuary cities are there?

There is no comprehensive list, because as stated above, the policies followed in different jurisdictions can be different. Some of the nation’s largest cities make the list, like New York City, Los Angeles, Chicago, Philadelphia and Phoenix. It also includes smaller cities like Takoma Park, Maryland; Richmond, California; and Ashland, Oregon.

There are also dozens of designated sanctuary counties that also decline to cooperate with federal authorities on immigration matters. The differences in what actually defines a sanctuary city will present a challenge to Trump in enforcing his new executive action.

How do people in the country without documentation end up all over the country?

People who are apprehended by law enforcement while crossing into the country without proper documentation are returned to their country of origin immediately in some cases. But in others, like when a person is making an asylum claim, they are put into a database and given a court date to appear for an immigration hearing. Pervasive drug and gang violence in Mexico and Central America means many of those crossing the southern border have legitimate claims that their lives are threatened at home.

Because the system is so backlogged, the government can’t afford (nor does it have space) to house all of those people in detention centers while a court decides if they have a legal case to be granted asylum. So people are let go and can then move freely about the country, where they often travel to join friends or family. People are supposed to return for their court date, but many do not.

There are also millions of people in the country that originally entered with the proper documentation, but overstayed a visa so are now living in the U.S. without current papers.