This form is useful during an IRS tax exmaination or audit. It establishes a legal basis for the definitoin of sources of income that are subject to income taxes.

"Sec. 61. Gross income defined
(a) General definition
Except as otherwise provided in this subtitle, gross income means all income from whatever source derived, including (but not limited to) the following items:
(1) Compensation for services...; (2) Gross income derived from business; (3) Gains derived from dealings in property; (4) Interest; (5) Rents; (6) Royalties; (7) Dividends; [more items listed]" [26 USC § 61]

"Determination of sources of income - Table of contents
Sec. 1.861-1 Income from sources within the United States.
(a) Categories of income. Part I (section 861 and following), subchapter N, chapter 1 of the Code, and the regulations thereunder determine the sources of income for purposes of the income tax." [26 CFR § 1.861-1]

"Income tax
Sources of income
Determination, 26 § 861 et seq…
Within the U.S., 26 § 861" [Index of the United States Code]