The original epibatoi of ancient times were hoplites trained to fight on a moving bireme or trireme in times of war and police it during peace time. They tended be steady veterans able to work well as a team or on their own when required. They were usually few in number and as weight and space was at a premium on warships.
The new epibatoi fulfil one central tenet of modern Empyraen thought; the Kalshodar are big and spaceships are, by necessity, full of places too small for them to fit into. Cruisers were built with Kalshodar in mind and most of their spaces are scaled to their bulk but not all. They were built by dwarves and humans for dwarves and humans because warriors spend little time in the magazines or drive spaces.
The epibatoi are the elite of the human soldiery; technically being both Navy and Army personnel, they are recruited from both branches so you can find ex-pilots and soldiery engineers among their number and they bring their earlier skills with them.
Much better armoured and equipped than their colleagues, epibatoi wear fully enclosed and atmospherically sealable impact armour. They train to fight in tight spaces and the desperate conditions of being boarded by the enemy. Naturally, a lack of atmosphere and heavy assault fire are to be expected.
Their weapons are also designed to be ship combat safe; using disabling komos rounds, non explosive ammunition and love up close and personal 'wetwork' in order to quickly eliminate boarders. On board ship or underground, epibatoi are the deadliest and most versatile soldiers humanity can produce.