Simple holder for the bristle discs / brushes, these can be found under the name bristle brush for Dremel, which you can buy on Amazon. The tray allows you to store up to 12 standard disc holder with a 3.2mm shaft and up to 10 bristle brush or screws in the middle compartment. I bought some extra M2.5*20mm screws to be able to use 5 to 8 bristle brush on one holder, for larger surface areas. Have fun making!