The GRAVELLY 18 is three packs in one: It'll securely "piggyback" to your main bag adding 18 more liters of capacity for critical gear. Or attach the GRAVELLY 18 to the frame and use the compression straps and Overload Shelf as a meat hauler while also carrying the essentials with you. Lastly, with stowable straps, it works as a stand-alone pack for a quick stalk or glassing session.