This is a flange for connecting your spray booth to a 5 gallon bucket with water to catch the particulates. I don't remember where I saw this idea but if you don't have an available window, you can put some water in a 5 gallon bucket, add a few holes for the air to escape, put some aquarium filter on the holes, and vent into the bucket. The water captures the particulates and the filters help clean the air escaping the bucket.