Loft is the volume for a given weight of down. It is used to describe the quality of down, relating directly to the insulating performance. High loft down is warmer for a given weight than low loft down. Higher lofting down will fill a larger volume of space for a given weight. As a result, less down weight is required to fill each baffle in a sleeping bag. This translates to a lighter sleeping bag. The most common measure of loft is called Fill Power. 850 Fill Power implies that one ounce of this down will loft to fill a volume of 850in3.
650+ Loft Premium Down
In the Sea to Summit Trek range, 650+Fill Power Premium duck down is a high quality insulation that sets the benchmark for sleeping bags in this class.