Although auxillary services are vital to the mission of any army, most aren't actually assigned to a field army. At least 15% of any given combat unit being on detail in rear area services at any given time during the war. In U.S. service, U.S.C.T. regiments were usually deployed to guard rail depots, lines of communication and military installations in the rear. They usually became engaged in combat during Confederate raids behind the lines. Some Federal officers sent these units to the front lines, but others, like William T. Sherman, refused to have them associated with his army except in rear area guard duties.
The law Congress passed on Feb. 17, 1864, was designed to free able-bodied white men in the Confederate army from rear area service assignments. Senior reserves -- men ages 45-50 -- were called into service specifically to replace younger, able-bodied soldiers detailed for these duties. The same is true for free black men also called into service by this law. Literally thousands of free blacks were conscripted just like senior reserves, the only difference being that they were not organized into companies, battalions and regiments.
Typical Department assignments --
1) COMMISSARY - Collection, storage and transportation of food supplies and livestock.
2) ENGINEER - Construction of fortifications and field defenses, bridges, dams and roads.
3) MEDICAL - Services as nurses and hospital aides, cleaning bed clothes and cooking for patients. Some also worked in burial crews and medical transportation.
4) ORDNANCE - Repair, maintenance and delivery of arms, ammunition and equipment.
5) QUARTERMASTER - Military operations involving transportation by wheeled vehicles, railroad or boat, including maintenance and repair, plus collection and storage of fuel and related supplies. Acquisition, storage and delivery of clothing, supplies and equipment needed by the military. Construction projects for standard facilities, such as barracks, warehouses, hospitals, prisons and similar ready-made structures.
6) SIGNALS - Communication between fixed and mobile field stations, including setting up telegraph lines and keeping them in order.
In addition to these assigned to military departments, there were persons employed by businesses, manufacturers and mining operations under contract to the Confederate government. All of this is in addition to those attached to regiments as teamsters, cooks and laundresses, and those who remained at home to keep things going.