For example the Roman republican tactic was the Triplex acies formation. In the front were the hastati, in the middle the principes and in the rear the triari. So how do the generals and officers decide which soldiers would be in the first line, second, third, etc. Of the hastati, principes and the triari? Sorry for my bad English, hope you understood the question.
You'll be interested in this answer from u/Iguana_on_a_stick, also this one, as to how you'd get into each division of the Roman army. There is also the Ancient Warfare section of the FAQ, which should do for some more of your questions in this vein.