sorry about the messy stiff i was doing ten things when i typed this . they are for external water packs or a day pack . ospreys have the same idea and i learned that it was for either the day pack you can buy separately or a external water pack . i actually was at a store up in Damascus ( mro ) and was just talking and looking at stuff with jeff and we ran across the pack and he was showing me some accessories and that is how i found this out . have a good day ... peace a nd love my friends and hike safely . the further shuttle Appalachian ... Donald " grateful " ballard