Here is a description of the medical categories from a Canadian source:
OFFICIAL HISTORY OF THE CANADIAN FORCES IN THE GREAT WAR 1914-19
THE MEDICAL SERVICES
(pp. 211-12)
For the purpose of ascertaining the physical condition of each soldier and his value as a reinforcement a system was established early in 1917 by which men were assigned to groups according to their fitness for service. Five medical categories were created, A, B, C, D, E, to include, respectively, men who were fit for general service; fit for certain kinds of service; fit for service in England; temporarily unfit but likely to become fit after treatment; and all others who should be discharged.
Category A was divided into four classes 1, 2, 3, 4, which contained respectively: men who were fit for active service in respect of health and training; men who had not been in the field but only lacked training; casualties fit as soon as they were hardened by exercise; and boys who would be fit as soon as they reached 19 years of age.
Category B was likewise subdivided into four groups, to include men who were fit for employment in labour, forestry, and railway units; men who were fit for base units of the medical service, garrison, or regimental outdoor duty; men capable of sedentary work as clerks; or skilled workmen at their trades. In Category C were placed men fit for service in England only.
In Category D were all men discharged from hospital to the command depot, who would be fit for Category A after completion of remedial training; and there was a special group to include all other ranks of any unit under medical treatment, who on completion would rejoin their original category. Category E included men unfit for A, B or C, and not likely to become fit within six months. It was a general rule that a soldier could be raised in category by a medical officer but lowered only by a board.
A commanding officer could, however, raise a man in Category A from second to first group, since training alone and not medical treatment was involved. All soldiers of low category were examined at regular intervals and new assignments made.
It was the function of the medical services to assign recruits and casualties to their proper categories. In April, 1918, when the demand for men became urgent, an allocation board was set up for the duty of examining all men of low category, and assigning them to tasks that were suitable for their capacity.
See also:
http://1914-1918.invisionzone.com/fo...owtopic=154834