Reply #1: Just a little bit nuts - The income defining sections are clearly labeled

[papau]

INCOME TAXES - Section 861. Income from sources with the United States
(a) Gross income from sources within United States - The following items of gross income shall be treated as income from sources within the United States: (1) Interest, ... (2) Dividends, ... (3) Personal services, ... (4) Rentals and royalties - Rentals or royalties from property located in the United States or from any interest in such property, including rentals or royalties for the use of or for the privilege of using in the United States patents, copyrights, trademarks, secret processes and formulas, good will, trade brands, franchises, and other like property.

(b) Taxable income from sources within United States - From the items of gross income specified in subsection (a) as being income from sources within the United States there shall be deducted the expenses, losses and other deductions property apportioned or allocated thereto and a ratable part of any expenses, losses or other deductions which cannot definitely be allocated to some item or class of gross income. The remainder, if any, shall be included in full as taxable income from sources within the United States. In the case of an individual who does not itemize deductions, an amount equal to the standard deduction shall be considered a deduction which cannot definitely by allocated to some item or class of gross income.