26 U.S.C. § 893: Compensation of employees of foreign governments or international organizations

such employee is not a citizen of the United States, or is a citizen of the Republic of the Philippines (whether or not a citizen of the United States); and in the case of an employee of a foreign government, the services are of a character similar to those performed by employees of the Government of the United States in foreign countries; and in the case of an employee of a foreign government, the foreign government grants an equivalent exemption to employees of the Government of the United States performing similar services in such foreign country. The Secretary of State shall certify to the Secretary of the Treasury the names of the foreign countries which grant an equivalent exemption to the employees of the Government of the United States performing services in such foreign countries, and the character of the services performed by employees of the Government of the United States in foreign countries. any employee of a controlled commercial entity (as defined in section 892(a)(2)(B)), or any employee of a foreign government whose services are primarily in connection with a commercial activity (whether within or outside the United States) of the foreign government.