The filter removes any residual dust not retained by the filter bag. It ensures the air returned to the room is clean and it protects the motor against dust. It filters particles down to 1 micron in size. (For situations or applications requiring HEPA filtration use 102253) It is compatible with CT-26, CT-36 and CT-48 extractors, helping to keep your Festool dust extractor running clean and strong. (Not suitable for mobile dust extractors with AUTOCLEAN function).