The Batmen were the 18th Century version of the Squires from the Middle-Ages. As such, they were only assigned to experienced high-ranking officers/knights. Batmen poured at feasts, they tended the horses, cleaned the weapons, fetched meals, and delivered messages. Anything not important enough for the officer himself.

In exchange, the veteran officer would teach the batmen how to fight and how to command. They’ll practice at arms, the officer would pay for their expenses, and their horses, and the batmen would be close, seeing all decisions made by his officer, gaining experience in command. And most of them will be well equipped by the officer.

Cadets were expected to take care of their own belongins and also some of them were selected to become a Batman/Squire of a veteran soldier. Officers could easily travel up and down the food chain, so being a batmen first was a task many like.

If the Batman was not a soldier but a civilian, he would not be trained but would be tipically well paid and later on would become a personal servant.