The alien registration number (alien number or A-number) has nothing to do with little green beings in flying saucers. In this case, "aliens" refers to immigrants (and in some cases, nonimmigrants). Basically, everyone who enters the United States as a permanent resident gets an alien registration number. The alien registration card is most commonly known as a green card.
The seven-, eight- or nine-digit alien registration number, given to all permanent, noncitizen residents in the United States, appears on a range of U.S. Citizenship and Immigration Services documents, including the green card (alien registration card, alien resident card or permanent resident card).
Where is the Alien Registration Number?
The U.S. Citizenship and Immigration Services uses the alien registration number to track immigration files for immigrants. It is a unique seven-, eight- or nine-digit number assigned to every noncitizen who applies to become a permanent U.S. resident. The number appears on all official paperwork relating to the application for immigration and must be added to forms requesting immigration benefits. It also appears on both the front and back of a green card. Generally, nonimmigrant visitors to the United States don't get an alien registration number, although certain nonimmigrants who have been granted employment authorization or other benefits may get one.
What is the Alien Registration Card?
An alien registration card (Form I-551) is best known as a green card, but is also known as a resident alien card or permanent resident card. The USCIS gives a green card to all lawful permanent residents (those who are lawfully entitled to live and work permanently in the United States) for identification purposes. To get a green card, someone else (a sponsor or petitioner) usually files an immigrant petition on your behalf, although you can file for yourself in some cases. When the USCIS approves the immigration petition, you then file a green card application with USCIS or visa application with the U.S. Department of State. The process includes an interview and a biometrics appointment to provide fingerprints, photographs and a signature.
What is the I-94?
If you visit the United States and arrive via a land border, you need an I-94 form. Exceptions are U.S. citizens, returning resident aliens, aliens with immigrant visas and most Canadian citizens who are visiting or in transit. Air and sea travelers used to get I-94 forms during the admission process at the port of entry, but paper I-94 forms are no longer required. A paper I-94 form is still issued at land border points of entry. Anyone in the United States who needs to prove their legal visitor status, for example, to employers or universities, can access their I-94 information via the Customs and Border Protection website.