Star filler is a type of plastic insert in CAT5 and CAT6 cable which separates the individual stranded pair sets from each other while inside of the cable. It dramatically increases the thickness and density of the cable but reduces cross-talk and is commonly used only in higher frequency CAT6E and CAT6A cable, but is also sometimes seen in CAT5e. A major reason it isn't used widely in cable in the consumer world is that it is not only typically more expensive than regular cable, but it takes up more room, it is harder to bend, and very difficult to manually crimp compared to CAT cable without the star filler. In the IT world it is often used in cable that is deployed in noisy environments such as datacenters or machine shops.