U.S. Tax Guide for Aliens, IRS
https://www.irs.gov/pub/irs-pdf/p519.pdf
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Dividends
In most cases,
dividend income received from
domestic corporations is U.S. source income.
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The 30% Tax
Tax at a
30% (or lower treaty) rate applies to
certain items of income or gains from U.S. sources but only if the items are not effectively connected with your U.S. trade or business.
Fixed or Determinable Income
The 30% (or lower treaty) rate applies to the
gross amount of U.S. source fixed or determinable annual or periodic gains, profits, or income.
Items specifically included as fixed or determinable income are interest (other than original
issue discount),
dividends, dividend equivalent
payments (defined in chapter 2), rents, premiums, annuities, salaries, wages, and other compensation.