A more thorough explanation:

An Individual Taxpayer Identification Number (ITIN) is a unique nine-digit number assigned by the Internal Revenue Service (IRS) to individuals who are not eligible to obtain a Social Security number (SSN). This includes foreign nationals and undocumented immigrants who need to pay federal taxes. However, an ITIN does not provide a person with immigration status, work authorization, or eligibility for social security benefits.

For example, if a nonresident alien is required to file a U.S. tax return, they can apply for an ITIN to fulfill their tax obligations.

To apply for an ITIN, a person must meet three requirements:

• Not eligible to obtain an SSN
• Required to furnish a federal tax identification number or file a federal tax return
• Belong to one of the eligible groups, such as nonresident aliens, U.S. resident aliens, or dependents/spouses of U.S. citizens/resident aliens

To apply for an ITIN, a person must fill out and submit a Form W-7 along with their federal income tax return, proof of identity, foreign nationality status, and residency. They can apply by mail, in person at an IRS Taxpayer Assistance Center, or with the help of an Acceptance Agent or Certified Acceptance Agent authorized by the IRS.

An ITIN will expire if its holder does not file a tax return for three consecutive years. However, it can be renewed by submitting a renewal application to the IRS.

Overall, an ITIN is a way for individuals who are not eligible for an SSN to fulfill their federal tax obligations.