are recommended for such appointment by their respective Governors or, in the case of the District of Columbia, the commanding general of the District of Columbia National Guard;

are recommended for such appointment by the Secretary of the Army or the Secretary of the Air Force;

have had at least 10 years of federally recognized commissioned service in an active status in the National Guard;

are in a grade above the grade of brigadier general;

are determined by the Chairman of the Joint Chiefs of Staff, in accordance with criteria and as a result of a process established by the Chairman, to have significant joint duty experience;

are determined by the Secretary of Defense to have successfully completed such other assignments and experiences so as to possess a detailed understanding of the status and capabilities of National Guard forces and the missions of the National Guard Bureau as set forth in section 10503 of this title;

have a level of operational experience in a position of significant responsibility, professional military education, and demonstrated expertise in national defense and homeland defense matters that are commensurate with the advisory role of the Chief of the National Guard Bureau; and

possess such other qualifications as the Secretary of Defense shall prescribe for purposes of this section.

An officer appointed as Chief of the National Guard Bureau serves at the pleasure of the President for a term of four years. An officer may be reappointed as Chief of the National Guard Bureau.

Except as provided in section 14508(d) of this title, while holding the office of Chief of the National Guard Bureau, the Chief of the National Guard Bureau may not be removed from the reserve active-status list, or from an active status, under any provision of law that otherwise would require such removal due to completion of a specified number of years of service or a specified number of years of service in grade.

a principal advisor to the Secretary of Defense, through the Chairman of the Joint Chiefs of Staff, on matters involving non-federalized National Guard forces and on other matters as determined by the Secretary of Defense; and

the principal adviser to the Secretary of the Army and the Chief of Staff of the Army, and to the Secretary of the Air Force and the Chief of Staff of the Air Force, on matters relating to the National Guard, the Army National Guard of the United States, and the Air National Guard of the United States.

The Chief of the National Guard Bureau shall be appointed to serve in the grade of general.

The Secretary of Defense shall designate, pursuant to subsection (b) of section 526 of this title, the position of Chief of the National Guard Bureau as one of the general officer and flag officer positions to be excluded from the limitations in subsection (a) of such section.

When there is a vacancy in the office of the Chief of the National Guard Bureau or in the absence or disability of the Chief, the Vice Chief of the National Guard Bureau acts as Chief and performs the duties of the Chief until a successor is appointed or the absence or disability ceases.

When there is a vacancy in the offices of both the Chief and the Vice Chief of the National Guard Bureau or in the absence or disability of both the Chief and the Vice Chief of the National Guard Bureau, or when there is a vacancy in one such office and in the absence or disability of the officer holding the other, the senior officer of the Army National Guard of the United States or the Air National Guard of the United States on duty with the National Guard Bureau shall perform the duties of the Chief until a successor to the Chief or Vice Chief is appointed or the absence or disability of the Chief or Vice Chief ceases, as the case may be.