This filter is the one located inside the cyclone. While your filter is washable, over time they may deteriorate. Which is why it is best to replace them when they do. Also keeping a spare is always handy.

This is a high quality after-market part that is a great alternative to the Genuine version. 

Will fit all versions of the DC28c and the DC39 vacuums. While it may look like other filters, it will only fit the models stated.