DS stands for "Drop Stitch" and refers to the floor inside the kayak that you sit on. The DS floor can be inflated to a very high pressure. This helps both with the rigidity of the kayak (you don't need a backbone) and with the aerodynamics of the hull (the water slides more easily under the kayak).
Advanced Elements are going to be releasing the DS floor for sale later this year, as an accessory for the Advanced Frame and Advanced Frame Convertible inflatable kayaks.
