What is the legal test used to determine whether an employee is entitled to official immunity?

The Westfall Act provides a procedure by which the federal government determines whether an employee is entitled to immunity. When a current or former federal employee is sued and the employee believes that he is entitled to official immunity, he is instructed to "deliver ... all process served upon him ... to his immediate supervisor" or other designated official, who then "furnish[es] copies of the pleadings and process therein to the United States attorney for the district embracing the place wherein the proceeding is brought, to the Attorney General, and to the head of his employing Federal agency." 28 U.S.C. 2679(c). The Attorney General then determines whether "the defendant employee was acting within the scope of his office or employment at the time of the incident out of which the claim arose." Id. 2679(d)(1). If so, the Attorney General issues a "scope certification," which "transforms an action against an individual federal employee into one against the United States." Hui v. Castaneda , 559 U.S. 799, 810, 130 S.Ct. 1845, 176 L.Ed.2d 703 (2010). The "United States shall be substituted as the party defendant," 28 U.S.C. 2679(d)(1), and the employee is released from any liability: "The remedy against the United States ... is exclusive of any other civil action or proceeding for money damages by reason of the same subject matter against the employee whose act or omission gave rise to the claim or against the estate of such employee. Any other civil action or proceeding for money damages arising out of or relating to the same subject matter against the employee or the employee's estate is precluded without regard to when the act or omission occurred." Id. 2679(b)(1).