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.

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Sources of Income

"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]

"Income from sources -

Within the United States, see section 861 of this title"
[Cross-reference under 26 USC § 61 in full version of Title 26]

"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]

"Income from sources inside or outside U.S., determination of sources of income, 26 CFR 1 (1.861-1--1.864-8T)." [Index of the Code of Federal Regulations]