Waterproof: Fully seam-sealed. Cordura fabric. Waterproof YKK zipper; Crescent shape: Eliminate dead space in a backpack more efficiently than a packing cube; Top handle & zipper pull tab: Enables easy access. Top handle also allows you to hang your PackStack PRO; Lightweight & durable construction: Withstand scrapes and scuffs; External storage pocket: Store miscellaneous items. Can also be used to store your PackStack PRO when not in use; Two versions: One fits 40L+ backpacks and the other 60L+ backpacks; Two sizes: Tall and Short (see specifications below). Gives you the ideal space to pack smart. For example separate what is wet/dry, clean/dirty, food/non-food, liquids/non-liquids and essentials/non-essentials; Mix and match: Option to combine and stack several units