The main difference between the indoor and outdoor style has to do with how thick the soles are. Indoor styles typically have lightweight and easy soles that allow the user to have a superior range of motion, and they can walk across hard surfaces without making any clicking noises or leaving scuff marks. The indoor/outdoor designs have a sole that is a lot thicker and resembles that of an actual shoe. It makes it so that you can venture outside to walk the dog on cold days, but users have found that they often making clicking noises when walking across hard surfaces, and also can sometimes leave scuff marks.