In general, a partnership (foreign or domestic) that has income effectively connected with a U.S. trade or business (or income treated as effectively connected) must pay a withholding tax on the effectively connected taxable income that is allocable to its foreign partners. In most cases, a partnership determines if a partner is a foreign partner and the partner’s tax classification based on the withholding certificate provided by the partner. The tax rate for such withholding varies depending on whether the foreign partner is a corporation, in which case the rate is the highest rate of tax specified in IRC Sec. 11(b). In case of foreign partners that are not corporations, the rate is the highest rate of tax specified in IRC Sec. 1. Note: Currently (2-16-2023), the withholding tax rate for effectively connected income allocable to non-corporate foreign partners is 37%, and 21% for corporate foreign partners.
See the full IRS article here: Withholding Tax on Foreign Partners’ Share of Effectively Connected Income – IRC Section 1446