What was the difference between the engaged and fighting men in a battlefield?

I want to know what was the difference between the number of engaged and fighting men, in a battlefield, during 19th century warfare. Was it due to Napoleonic infantry tactics during the period?What was the difference between the engaged and fighting men in a battlefield?
Look at it from a contemporary standpoint and you will understand it. It takes 10 men to keep one on the front lines. Much of that is simply in logistics, ammunition, food, and things like that, but still more are in medical and other fields. that is simply the way that the military works.

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