@pk, this response is incorrect. Having an SSN does not mean that a W-9 is the correct form to complete and submit to the requester. In many cases a W-8BEN is the correct form for a person with an SSN.
Per Part II of the Form W-9, you are certifying that you are a US citizen or other US person. For individuals, a "US person" is a resident alien, meaning they are either a green card holder (GCH) or someone who qualifies as a US tax resident because they pass the substantial presence test (SPT). In addition, although a resident alien would typically have an SSN, there are situations where they would not (e.g., child accompanying their parent in US, but the child does not have work authorization).
For the situation described by @ChikakoS in the previous posts, a Form W-9 is the incorrect form because they have left the US and are no longer a US tax resident. If a W-9 was previously provided to the requester, a W-8BEN should have been submitted at the end of their US tax residency period.
In addition, for purposes of the SPT, days present in the US on a J visa are often exempt (i.e., not counted). As such, it is likely that @ChikakoS did not pass the SPT and was not a US resident alien. In that case, they should have filed a 1040NR tax return for the years they were in the US on a J visa and a W-8BEN should have been used the entire time.
If, based on @ChikakoS's specific situation, the US days on a J visa are not exempt for SPT, they would have been a US resident alien for a period of time. In that case, a regular Form 1040 resident income tax return would be filed their years in the US and a W-9 could have been used.
There is, of course, also the possibility of a dual status tax return, but I won't get into that here.
