This is the gasket seal that is accessed by splitting the cyclone to suit the DC28c, DC33c, DC37, DC39 cylinder models.
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. Please do not buy on spec.