The Featherweight RDS Down Vest super-lightweight, while locking in warmth. It has a water-resistant outer combined with downproof lining and 90% RDS down - a must-have addition to your outerwear wardrobe. Fill Power Rating 600 - Higher fill power = warmer item. The measure of the loft (fluffiness). The higher the rating, the more air trapped per ounce of down allowing a higher level of insulation RDS Down - This product is Responsible Down Standard (RDS) certified by Control Union CUC-890403. RDS certified products contain down and feathers from farms certified to animal welfare requirements. Mountain Warehouse Featherweight II Mens Down Vest