"The Federal Vacancies Reform Act of 1998 (Vacancies Act) generally provides the exclusive means by which a government employee may temporarily perform the nondelegable functions and duties of a vacant advice and consent position in an executive agency. Unless an acting officer is serving in compliance with the requirements of the Vacancies Act, any attempt to perform the functions and duties of that office will have no force or effect. The Vacancies Act limits a government employee's ability to serve as an acting officer in two primary ways. First, the Vacancies Act provides that only three classes of people may serve temporarily in an advice and consent position. As a default rule, the first assistant to a position automatically becomes the acting officer. Alternatively, the President may direct either a senior official of that agency or a person serving in any other advice and consent position to serve as the acting officer. Second, the Vacancies Act limits the length of time a person may serve as acting officer: a person may serve either (1) for a limited time period running from the date that the vacancy occurred or (2) during the pendency of a nomination to that office. The Vacancies Act is primarily enforced when a person who has been injured by an agency's action challenges the action based on the theory that it was taken in contravention of the Act."
CRS Report for Congress, R44997
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html