What is the US alien number?

Excuse me, could you please clarify what exactly is meant by the "US alien number"? Is this referring to a specific identification number assigned to non-citizens residing in the United States? If so, could you provide more context or detail on what this number represents and how it is used? Is it related to immigration status, tax purposes, or perhaps something else entirely? I'm interested in learning more about this concept and how it fits into the broader context of US immigration and finance.