Four units with their main and sub units will comprise your party. The main unit automatically participating in the battles. When a main unit gets killed, its sub unit will be automatically become active in the battle. In Fortress of Chaos and Battle of Heroes, all party members will take part in a battle together. There are four groups of parties that can be set. You can check the main party on the first page of the "Set Party" screen. You can set and use the different units for two to four sub parties. You can choose one of the set party groups before the battles, and the same member can be set to four different parties. Your party has a frontline and backline. The frontline will engage in battle first. When they are killed, the backline will take its place.