This is one of the most important design aspect of any sleep system that connects around the sleeper, such as a sleeping bag or a quilt that connects with straps to a pad or itself. By definition, it is when the inner and outer shells of a sleep system are cut to different sizes so that all of your down fill can stay fully lofted around you even if you push against it or pull it tight. If the inner and outer shells are not cut with a differential, any pressure the sleeper puts on it will compress it, compromising warmth greatly. If you pull the sides of a quilt not cut with a differential, ALL of the down along that line across will flatten out. Pull the sides of a differential cut quilt and all the of the load is received by the smaller shell and the down stay fluffy, like it should be. I use either an external or an internal differential cut on all of my sleep systems except for the ones using Climashiled APEX, which doesn't require this due to it being sewn in as a continuous batt.