Each recruit to the Army had to be graded medically.  This grading was performed by National Service Medical Boards, which were composed of civilian Medical Practitioners acting on behalf of the Minister of National Service.

The categories were simplified from the 'B1', etc., system to a 'Grade' system in December 1917.  The medical categories were as follows:-

Grade I, generally equivalent to Category A, meant that the recruit had attained the full normal standard of health and strength and was capable of enduring the amount of exertion suitable to his age.  Recruits had to be free from any serious organic disease or deformity.

Grade II, generally equivalent to Categories B 1 and C 1, included those who, while not attaining the standard of Grade I, were able to stand a fair amount of physical strain and were likely to improve if trained.  Men in this Grade had to be able, when trained, to march six miles with ease.  They had to have fair sight and hearing and have average muscular development.

Grade III, generally equivalent to Categories B 2 and C 2, B 3 and C 3, included men who, from any cause, were not likely to be suitable for military training for combatant service and were fit only for clerical and other sedentary jobs.  This Grade included those who presented marked physical deformities such as badly deformed toes or flat feet. 

Grade IV, generally equivalent to rejection under the former classification, included men who were clearly found by a unanimous Board to be permanently and totally unfit for any form of military service.

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Sources:
The Recruiting Code (pub. Ministry of National Service, London, issued 1918)
Report upon the physical examination of men of military age by National Service Medical Boards from November 1st, 1917- October 31st, 1918, Volume I prepared by the Ministry of National Service (pub. HMSO, London, 1920)