The U.S. income tax and estate tax both function under a system with only two possibilities, a taxpayer is either a U.S. person/domiciliary or not a U.S. person, referred to as a non-resident alien (“NRA”). There is no middle ground, it is an all or nothing system. Since U.S. tax law is based on this distinction, the first step in any analysis is determining the correct category.
The U.S. laws do not define who is an NRA, rather, they define who is a U.S. person, or for estate taxes a U.S. domiciliary. Therefore, NRAs will be anyone who does not meet the definition of a U.S. person or domiciliary. Due to these rules, before we can decide how a foreigner will be taxed in the U.S., we must first make sure they are in fact a foreigner under the laws.