He was detailed extra duty away from his company serving with the Pioneer Corps. for some type of engineering duty, repairing bridges, roads, building fortifications, etc.
He may have been an Artificer or a common laborer.
For information on their work, see:
http://www.google.com/search?client=safari&rls=en&q=confederate+pioneer+corps&ie=UTF-8&oe=UTF-8