The Scoop Free 3 pack of litter refill trays go with the Scoop Free Ultra Self-Cleaning Litter Box, which is Purr-fect for our Precious Puddy Cat. It is simple to setup and maintain and it works great. I love that we can leave it for weeks and the litter box takes care of itself. The self cleaning unit is great, and these sensitive crystal litter trays are great for our senior cat with longstanding medical issues. Why didn't I get both of these earlier?