What the checker looks at
- Length and characters. Nine digits. Anything else is IRS TIN Matching code 1. Leading zeros matter: "012345678" is a valid format that spreadsheets silently turn into "12345678".
- SSN structure. The area (first three digits) can't be 000, 666 or 900–999; the group (middle two) can't be 00; the serial (last four) can't be 0000. Since 2011 the SSA assigns numbers randomly, so nothing else about the area number tells you where or when it was issued.
- ITIN structure. Nine digits beginning with 9, with a fourth-and-fifth-digit group in the ranges the IRS assigns to ITINs (50–65, 70–88, 90–92, 94–99). An ITIN holder is an individual; the W-9 should carry a person's name.
- EIN prefix. The first two digits of an EIN come from a fixed set of campus and internet-assignment prefixes. A prefix outside that set (for example 00, 07, 08, 09, 17, 18, 19, 28, 29, 49, 69, 70, 78, 79, 89, 96, 97) has never been issued.
Passing this check means the number could exist. Whether it does, and whether it goes with the name, is what IRS TIN matching answers.