If the primary or secondary taxpayer's application for an ITIN is rejected, the IRS assigns the tax return an Internal Revenue Service Number (IRSN) so the return can be forwarded for processing.
The IRS will also assign an IRSN if there is no ITIN or SSN on a tax return, payment, or other document, or if the one used on the return cannot be located or matches another taxpayer.
The IRSN looks like an ITIN since it's in the same 9-digit format and begins with "9", but it's an internal identifying number the IRS uses for its files until the taxpayer is assigned an SSN or ITIN, or until their SSN or ITIN is found. The IRS will use the IRSN on any correspondence with the taxpayer, and likewise the taxpayer should reference the IRSN in their correspondence with the IRS, including with payments.
An IRSN is issued to the primary taxpayer and also to the secondary taxpayer if they have a Schedule SE in the tax return. Dependents aren't assigned an IRSN.
An IRSN may also be referred to in IRS correspondence as a Temporary Taxpayer Identification Number. An IRSN cannot be used to file a tax return.
For an IRSN, the fourth and fifth-digits are 01-10, 17-19, 28-29, 37-39, 49, 66, or 89 and correspond to an Internal Revenue Service Campus (IRSC):
| IRSC | Middle digits |
| Brookhaven | 01, 19 |
| Cincinnati | 02, 17 |
| Memphis | 03, 49 |
| Ogden | 04, 29 |
| Philadelphia | 05, 28 |
| Austin | 06, 18, 66 |
| Atlanta | 07, 37 |
| Andover | 08, 38 |
| Kansas City | 09, 39 |
| Fresno | 10, 89 |
For comparison, here are the ranges of ITIN and ATIN middle digits:
| Number | Middle digits |
| ITIN | 50-65, 70-88, 90-92, 94-99 |
| ATIN | 93 |