A foreign national is considered to be a US Resident Alien for tax purposes if he/she meets the "Green-Card Test" (holds an Alien Registration Card) or passes the Substantial Presence Test (SPT). The SPT is a method of calculation used by the Internal Revenue Service which consists of counting the days physically present in the US. A Resident Alien is taxed in the same manner as a US Citizen. However, a Resident Alien is not considered a US Citizen or Permanent Resident for immigration purposes.
Note:For information on what documentation is required for payments to Resident Aliens: