Air is exhausted from the top of the VAC-35 dust extractor. This kit prevents dust from escaping into the workspace in case of a defective filter cloth. It also allows you to check whether the filter cloth inside the VAC-35 is leaking. If there is more than a small layer of dust inside the final filter, replace the vacuum bag (ref. 6011-00) immediately. The final filter provides a cleaner and quieter working environment.