I believe this refers to special enlistment, authorisation for which had to be obtained.
It was for when a recruit did not meet the age requirements (too old or too young), or the physical requirements (too small), but would either get older and bigger, or fitter and make a good soldier with training.
It also applied when the recruit was married, a widower with dependent children, or foreign! It should always have been used for boys wishing to join up.
"Cookery is the art of preparing and softening food by the action of fire, so as to render it fit for digestion" - Instructions to Military Cooks in the Preperation of Dinners at the Instructional Kitchen, Aldershot, 1878.