10 U.S.C. § 601 : US Code - Section 601: Positions of importance and responsibility: generals and lieutenant generals; admirals and vice admirals

      (a) The President may designate positions of importance and
    responsibility to carry the grade of general or admiral or
    lieutenant general or vice admiral. The President may assign to any
    such position an officer of the Army, Navy, Air Force, or Marine
    Corps who is serving on active duty in any grade above colonel or,
    in the case of an officer of the Navy, any grade above captain. An
    officer assigned to any such position has the grade specified for
    that position if he is appointed to that grade by the President, by
    and with the advice and consent of the Senate. Except as provided
    in subsection (b), the appointment of an officer to a grade under
    this section for service in a position of importance and
    responsibility ends on the date of the termination of the
    assignment of the officer to that position.
      (b) An officer who is appointed to the grade of general, admiral,
    lieutenant general, or vice admiral for service in a position
    designated under subsection (a) or by law to carry that grade shall
    continue to hold that grade - 
        (1) while serving in that position;
        (2) while under orders transferring him to another position
      designated under subsection (a) or by law to carry one of those
      grades, beginning on the day his assignment to the first position
      is terminated and ending on the day before the day on which he
      assumes the second position;
        (3) while hospitalized, beginning on the day of the
      hospitalization and ending on the day he is discharged from the
      hospital, but not for more than 180 days;
        (4) at the discretion of the Secretary of Defense, while the
      officer is awaiting orders after being relieved from the position
      designated under subsection (a) or by law to carry one of those
      grades, but not for more than 60 days beginning on the day the
      officer is relieved from the position, unless, during such
      period, the officer is placed under orders to another position
      designated under subsection (a) or by law to carry one of those
      grades, in which case paragraph (2) will also apply to the
      officer; and
        (5) while awaiting retirement, beginning on the day he is
      relieved from the position designated under subsection (a) or by
      law to carry one of those grades and ending on the day before his
      retirement, but not for more than 60 days.

      (c)(1) An appointment of an officer under subsection (a) does not
    vacate the permanent grade held by the officer.
      (2) An officer serving in a grade above major general or rear
    admiral who holds the permanent grade of brigadier general or rear
    admiral (lower half) shall be considered for promotion to the
    permanent grade of major general or rear admiral, as appropriate,
    as if he were serving in his permanent grade.
      (d)(1) When an officer is recommended to the President for an
    initial appointment to the grade of lieutenant general or vice
    admiral, or for an initial appointment to the grade of general or
    admiral, the Chairman of the Joint Chiefs of Staff shall submit to
    the Secretary of Defense the Chairman's evaluation of the
    performance of that officer as a member of the Joint Staff and in
    other joint duty assignments. The Secretary of Defense shall submit
    the Chairman's evaluation to the President at the same time the
    recommendation for the appointment is submitted to the President.
      (2) Whenever a vacancy occurs in a position within the Department
    of Defense that the President has designated as a position of
    importance and responsibility to carry the grade of general or
    admiral or lieutenant general or vice admiral or in an office that
    is designated by law to carry such a grade, the Secretary of
    Defense shall inform the President of the qualifications needed by
    an officer serving in that position or office to carry out
    effectively the duties and responsibilities of that position or
    office.