This add on pack creates an additional 4L of storage to the rear of the pack. It attaches via webbing at the top, and through the nylon web at the bottom. It's best for light jackets and layers, not heavy bulky items. 

Additionally, it comes with the shock cord and trekking pole hardware already attached.

As an added bonus, you're not just adding another pack to your gear, you're creating a pack that can be used for multiple purposes.

Note: the Endurance Pack pictured does not come with the add on bag, it must be purchased separately.