Boarding is a now defunct mechanism that was a means of acquiring new ships from defeated enemies using marines, which no longer have a special purpose and are only a trade commodity.
After a battle, one of the disabled enemy ships may come online, with a certain number of surviving crew. The player can make a boarding attempt, destroy the ship, or let it go. If the player chooses to board, the player's marines attack the ship and attempt to kill all the defending crew. If they succeed, the ship is captured.