This is undoubtedly a reference to the reenlistment of Union soldiers when their original terms of enlistment were expiring. If a sufficient number of the men reenlisted, their units were redesignated as "Veteran Volunteers," and they would be given a furlough, 30 days I think. The men who refused to renlsit as veteran volunteers would be "non-veterans" and would reamin on whatever onerous duty the powers that be could devise.