The PF-2 filters are optional  placed under the Black Berkey filters in the lower chamber and are used to filter arsenic, fluoride, and heavy metals in the water. Enev though the Black Berkey filters are effective against these pollutants, a high levels of these contaminants can saturate them before their normal lifespan. To determine if you need these filters in your system, check the analysis of the water you will be filtering. More details on these filters can be found by consulting the product sheet.