The UV treatment is identically present in all UVION devices, because only a specific wavelength (254nm) of the UV light eliminates pathogens reliably.
First the water flows through the UV chamber. The UV light kills all possibly contained bacteria, viruses, legionella and other germs in the pool water immediately. The water is now germ-free. However, new pathogens get into the pool constantly through bathing and the environment. In addition, various algae grow in the water. The warmer the water is, the faster algae and pathogens increase. Which is why, the water must flow through a second treatment stage.