This is an upgrade to the standard bivy extension sleeve that is primarily on the classic packs. all the top loading Alpine Machine & New Day Rising packs come with this feature. there is no extra cost. So, if your buying an alpine pack there is no additional fee.
This is similar to a standard bivy extension. Common features are dual drawcords, 12" tall and both are cut oversized so they can be pulled down over your pack for easy packing.
The differences between the two are the roll top version is built out of waterproof X-Pac VX21 fabric, the roll top can be used without a lid and still have a weatherproof pack closure. The Roll Top is standard on the both the Alpine Machine and New Day Rising packs, optional on all other top loading packs.