In some games, several sides may be very similar to each other, while being very different from other sides in the same game. For instance, the game might have several sides that are different tribes of barbarians, but they are more like each other than, say, Romans. These similar sides can be given the same side class. Units may then be restricted to be usable only by the sides in a particular class. (Note that this is different from tech level, which allows units to be used by any side that has managed to acquire a sufficient tech level.)