Definition:
Salaries, wages, bonuses, or any other pay for personal services (referred to collectively as wages) paid to nonresident alien employees are subject to graduated withholding in the same way as for U.S. citizens and residents if the wages are effectively connected with the conduct of a U.S. trade or business. Also, the 30% withholding does not apply to pay for personal services performed as an employee for an employer if it is effectively connected with the conduct of a U.S. trade or business and is specifically exempted from the definition of wages.
Chapter 4 withholding does not apply to these payments. See Page 35