An alien registration number, also known as an "A-number," is a designation that some aliens receive from the USCIS Immigration Service. A person who applies for a status adjustment is more likely to receive an A-number.
Alien numbers have either eight or nine digits, and similar to a Social Security number, once a person receives one, he has it for life. The alien registration program was initiated in 1940 to keep track of every person in the United States who was not a citizen. At the time, the law was implemented in the interests of national security, and it also mandated that the Immigration and Naturalization Service register and fingerprint all aliens at least 14 years of age who were either presently in the country or who were entering the country.