Soldiers in the army have usually something like a kit-bag, see sack, weekend bag or duffel bag. This not to use in the combat field but to take all their clothing and gear from army base to army base or from their barracks to home during weekends. These bags are usually very simple. In case of a duffel bag it is usually a one compartment bag with rings and a clip to close it instead of zippers. Most duffel bags have just one strap to throw over your shoulders. The US army duffel bag however is one of the few designs with two straps so that it can be worn as a (improvised) backpack. To improve the possibility of carrying the duffel bag as a backpack the bag has a rectangular shape instead of a cylindrical shape. A round shape will flip sidewards all the time when you have it on your back. A flat shape will be much more stable.