The USMC intermediate sleeping bag was designed to be used with or without the 3-season bag and bivy to create a sleeping bag system for the U.S. Marine Corp.  When used alone, the intermediate sleeping bag is rated to 15 degrees but may be used down to -20 when used inside the system making this a versatile bag for 3 and 4 seasons.  The intermediate bag was also made larger in order to comfortably accommodate the inner bag making this a great bag when extra room is needed.  As with all military sleeping bags, they are rated when the soldier is wearing a medium weight base layer.  When not in the field, it is always recommended to use the military mesh storage bag in order to allow the insulation to keep its loft and heat retention.  Building a good sleep system will allow you the options to always be ready.  So, get out and enjoy the outdoors.