Immigration law is complex and is governed by several sources. The immigration statute is called the Immigration and Nationality Act (INA). The INA was passed by congress in 1952 and has been compared as second in complexity only to the United States tax code. There are sections in the Act that describe how a person might qualify for admission on a temporary or permanent basis; sections about why a person might face deportation from the United States; and parts about defenses to deportation, like asylum. Many forms of immigration benefits and relief involve both rigid statutory criteria and the exercise of discretion. Decisions about whether a person should be admitted to, expelled from or granted relief in the United States are made primarily by employees of the Department of Homeland Security (DHS), Department of State (DOS) or Department of Justice (DOJ).
Shoba Wadhia, Immigration Enforcement and the Future of Discretion, 23 Roger Williams U. L. Rev. 353 (2018).