the bottom screws onto the Sawyer bag, bottles, etc. Lest you confuse clean and dirty, the top doesn't screw onto anything. There is a little fitting for camelback hoses, the included straw, or your mouth. I would much prefer to be able to screw something on both ends. Clean and dirty ends could be separated some other way— a red and green end, for example, or write DIRTY on your dirty water bag. Presently you can go Sawyer Bag to camelback hose, or just prop the bag and filter upsidedown to drain into a pot or Nalgene, as I have been doing. You can't go from one sawyer bag to another, though, in a sealed, leakproof, tipoverproof manner. This is the only thing that bugs me, as I don't like it when impediments are built into a design to save people from themselves (other than in complex systems where an average user can't really tell what's going on).