Which number should we use on taxes? ITIN or work authorization number (9 digit number, but not SSN).

Use the ITIN if you don't have an SSN.   An Individual Taxpayer Identification Number (ITIN) is a tax processing number issued by the Internal Revenue Service. The IRS issues ITINs to individuals who are required to have a U.S. taxpayer identification number but who do not have and are not eligible to obtain, a Social Security number (SSN) from the Social Security Administration.  

If you're required to file a federal return for 2020, you can use Form 1040 (U.S. Individual Income Tax Return) if any of these are true:

• You're a U.S. citizen;
• You're a resident of the U.S. or Puerto Rico;
• You have a Permanent Resident Card ("green card");
• You're a nonresident alien married to a U.S. citizen/resident alien with whom you will file a joint return; or
• You meet the substantial presence test, which means you were physically present in the U.S. for at least 31 days in 2020 and at least 183 days in 2018 through 2020 according to this formula:

2020 days + (2019 days)/3 + (2018 days)/6 >= 183   For detailed example - click here

Those who don't pass the substantial presence test are considered nonresident aliens, nonresident aliens also include:

• Foreign nationals
• Anybody who is not a U.S. citizen, U.S. or Puerto Rico resident alien, or doesn't hold a green card
• A person who has income from or participates in a U.S.-sourced trade or business

Unlike Form 1040, there's no minimum income threshold for filing a 1040-NR. In other words, nonresident aliens with U.S. income, regardless of amount, are required to file a 1040-NR.

Related Information:

• Does TurboTax handle Form 1040-NR for nonresident aliens?   No, but this link has information about Sprintax, where you can file a Form 1040-NR if needed.