The name says it all: Nimbus means it has a 3D Tepex Nimbus style framesheet: the best-fitting, most comfortable and durable frame on the market. Access means a panel loading system that makes packing and unpacking a breeze. FZ means it is leaner and meaner than most. Light Packs, the pack category that means it is designed for the backpacker that travels light (not ultralight). The main differences between the Nimbus Light Packs and the Nimbus Ultralight Packs are the beefier fabrics and the addition of a hip belt with an exoskeleton of HDPE plastic that supports more weight without sagging, and an external lid. Finally, 3800 means 3800 cubic-inch capacity.