A green card, known officially as a permanent resident card, is an identity document which shows that a person has permanent residency in the United States. Green card holders are formally known as lawful permanent residents (LPRs). As of 2019, there are an estimated 13.9 million green card holders, of whom 9.1 million are eligible to become United States citizens. Approximately 18,700 of them serve in the U.S. Armed Forces.

The Selective Service System (SSS) is an independent agency of the United States government that maintains a database of registered U.S. citizens and other U.S. residents potentially subject to military conscription. Although the U.S. military is currently an All-Volunteer Force, registration is still required for contingency planning and preparation for two types of draft: a general draft based on registration lists of people assigned male at birth, aged 18–25, and a special-skills draft based on professional licensing lists of workers in specified health care occupations. In the event of either type of draft, the Selective Service System would send out induction notices, adjudicate claims for deferments or exemptions, and assign draftees classified as conscientious objectors to alternative service work. All male U.S. citizens and immigrant non-citizens who are between the ages of 18 and 25 are required by law to have registered within 30 days of their 18th birthdays, and must notify the Selective Service within ten days of any changes to any of the information they provided on their registration cards, such as a change of address. The Selective Service System is a contingency mechanism for the possibility that conscription becomes necessary.