Adding slots can increase the extra attributes of the equipments. Any of the weapons, helmets, armors, trousers, and cloaks can be strengthened through adding slots on them. The weapons can be strengthened three times, however, the helmets, armors, trousers and cloaks can be strengthened twice.
There are some equipments that cannot be strengthened. So that when it have tips that “red crystals are available to add slots”, you can strengthen them by purchasing crystals and adding slots on them.