The best illustration of how the SF works (whether with their cartridge or any of the permanent trays) is to watch the video at their website --
www.scoopfree.comUnlike littermaid, the SF doesn't have a provision for seperately emptying the waste trap only. One can do an occasional "rake out" from under the lid (it's pretty easy, and what many of us do) that extends the time between changes -- but there's no way to just dump that trap by itself and replace.
Hope this helped!