[1] Obtaining such clearance requires, in part, a Single Scope Background Investigation (SSBI) which is conducted under the manuals of the U.S. Defense Counterintelligence and Security Agency.
Individuals with Yankee White clearance undergo extensive background investigation.
The Yankee White clearance includes requirements of U.S. citizenship and unquestionable loyalty to the United States.
[1] The initial directive detailing the checks required was DoD Directive 5210.55, "Selection of DoD Military and Civilian Personnel and Contractor Employees for Assignment to Presidential Support Activities" issued on July 6, 1977.
[2] DoD Instruction 5210.87 contains detailed information for the implementation of the policy, assignment of responsibilities and prescription of procedures to be followed.