There is a good description of all the flight crew specific features on the site, of which there are many. That outside compartment is seperated from the main compartments inside. It is designed to carry your walk around items like vests, gloves, hat, flashlight etc. It is also a quick easy spot to drop in water bottles, snacks etc. The cool aspect they added in is that it is seperated from the rest of your clothes so if your walk around is in the rain or snow, your wet stuff will not get the dry stuff inside the main case wet. Same goes for sweating watter bottles. That compartment is not sealed like the main compartment which has a tongue and groove seal that you can spray with a hose and it stays dry. The small compartment is desighned to let wet stuff evaporate.

These guys really put a lot of design work into these for our specific needs.