This is the gasket seal that is accessed by splitting the cyclone to suit the DC40 and US/Canada DC42 models only.
Sometimes these degrade and cause cyclone leakage and thus suction loss. Sometimes when you split a cyclone for cleaning, the old one is unusable rendering the cyclone effectively scrap.
However, we now sell the replacement cyclone gasket seals here.
Please be sure to perform a visual check with your old one to be certain you are buying the correct one. They all look pretty similar across model variants.