"US person" includes US citizens, green-card holders, and US-resident aliens (substantial presence test). It also includes US-formed entities (LLCs, C-corps, S-corps, partnerships).
An "entity" means a company, corporation, trust, partnership, or any organization (LLC, GmbH, Ltd, SARL, etc.). Basically, anything other than a single human filing personally.
A treaty can reduce or eliminate the default 30% US withholding. Most major economies (UK, Canada, Germany, France, India, Australia, Japan) have one.
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