I-Dry means it's wicking the sweat away. HP stands for High Performance. With breathable fabric it's ideal for casual rounds or a round at the Worlds. This version offers a new 3/4 Sleeve that has all the warmth of a sweatshirt without the bulk. The 3/4 Sleeve Shirt will keep you warm without inhibiting your throw or putting motions. Too hot? Not a problem the 3/4 Sleeve Shirt is extremely light weight and will fit into most disc golf bag pockets.