It's a really popular modification which allows You to put some wider documents in the back of the bag. By adding it we create an useful additional space. 

On the photos You can see this modification used with the Big River bag.

It can be used only in chosen models. If You want to see if it's available for the bag that You picked just enter the "Accessories" bookmark (next to the description) and see if "Back zipper pocket" is available. If so - just add it to Your cart.