IRS Form 4506:
Request for Copy of Transcript or Tax Form
Item 8(b) is a request for verification of nonfiling
IRS
Restructuring and Reform Act, Section 3707, 112 Stat. 778:
SEC. 3707. ILLEGAL TAX PROTESTER DESIGNATION.
(a) PROHIBITION.—The officers and employees of the Internal Revenue Service—
(1) shall not designate taxpayers as illegal tax protesters (or any similar designation); and
(2) in the case of any such designation made on or before the date of the enactment of this Act—
(A) shall remove such designation from the individual master file; and
(B) shall disregard any such designation not located in the individual master file.
(b) DESIGNATION OF NONFILERS ALLOWED.—An officer or employee of the Internal Revenue Service may designate any appropriate taxpayer as a nonfiler, but shall remove such designation once the taxpayer has filed income tax returns for 2 consecutive taxable years and paid all taxes shown on such returns.
(c) EFFECTIVE DATE.—The provisions of this section shall take effect on the date of the enactment of this Act, except that the removal of any designation under subsection (a)(2)(A) shall not be required to begin before January 1, 1999.
Internal Revenue Manual (I.R.M.), Section 4.19.17: Nonfiler Program
26 U.S.C. §6651: Failure to file tax return or pay tax -NOTES talks about "nonfilers"
TITLE 26 > Subtitle F > CHAPTER 68 > Subchapter A > PART I > Sec. 6651.