Officers may be selected for continuation on active duty pursuant to this section only for assignment to positions identified by the Secretary of the military department concerned for which vacancies exist or are anticipated to exist.

Before convening a selection board pursuant to section 611(b) of this title for purposes of selection of officers for continuation on active duty pursuant to this section, the Secretary of the military department concerned shall specify for purposes of the board the positions identified by the Secretary to which officers selected for continuation on active duty may be assigned.

An officer continued on active duty pursuant to this section shall remain on active duty, and serve in the position to which assigned (or in another position to which assigned with the approval of the Secretary of the military department concerned), for a total of not more than three years after the date of assignment to the position to which first so assigned.

An officer whose continued service pursuant to this section would otherwise expire pursuant to paragraph (1) may be continued on active duty if selected for continuation on active duty in accordance with this section before the date of expiration pursuant to that paragraph.

be discharged upon the expiration of the term of continued service; or

if eligible for retirement under another other provision of law, be retired under that law on the first day of the first month following the month in which the officer completes the term of continued service.