This adapter for Purrified Air filter is designed to allow use of the filter on both large and small domed-shaped litter boxes such as the Booda, Petmate and others. Depending on the size of your domed litter box, you will need to determine if you need the AC (Adapter with Cutout) model, or the AR (Adapter Full Round). Generally, the large boxes require the AR, while the smaller ones where the top of the entry way arc require the cutout. Free shipping in United States.