Let's say that a muslim state goes into battle with a kafir party. They then would give the kuffar these options: convert, pay the jizya (if applicable), or fight. If the kuffar chose the first or second, they're safe. If they chose the third, they'd be killed. Then where do captives come from?
Men of rank were often held for ransom. That would be a group of captives. The non-combatants assisting the opposition were taken as captives, such as slaves who did the cooking. Their women, their children. That is four different kinds of captives.