This jacket is part of the US Protective Combat Uniform (PCU) system that came to be in 2002. The PCU system is divided into seven layers that are combined with each other to protect against various weather conditions. The basic philosophy is that air trapped between layers of clothing will create the best form of insulation. It is not necessary to wear all seven layers at the same time. You pick and choose those that you need.
Level 6 (L6) includes the hardshell jacket and pants. They go on top of everything in rainy, sleety, shitty weather. Underneath you can add other layers depending on how cold it is.