The Field Ambulance was a mobile front line medical unit (it was not a vehicle). Most came under command of a Division, and had special responsibility for the care of casualties of one of the Brigades in the Division. Each Division had three Field Ambulances. The theoretical capacity of the Field Ambulance was 150 casualties, but in battle many would simply be overwhelmed by numbers. The Ambulance was responsible for establishing and operating a number of points along the casualty evacuation chain, from the Bearer Relay Posts which were up to 600 yards behind the Regimental Aid Posts, through the Advanced Dressing Station (ADS), to the Main Dressing Station (MDS). It also provided a Walking Wounded Collecting Station, as well as various rest areas and local sick rooms. The Ambulances would usually establish 1 ADS per Brigade, and 1 MDS for the Division.