Using a Practitioner PIN
If a return does not qualify for a Self-Select PIN, or if the taxpayer does not choose to use one, the Practitioner PIN enables the tax preparer to sign the return on behalf of the taxpayer. The taxpayer must give permission to the volunteer to sign the return on Form 8879, IRS e-file Signature Authorization. For more information, review Finishing the Return in the Volunteer Resource Guide (Tab 12).
Electronic transmission is not always possible for members of the Armed Forces who are signing joint returns. See Publication 3.