Indoor Training Never Looked This Good Castelli made this jersey for the rigors of indoor training. The 3D Mesh fabric allows maximum airflow for cooling while the 100% polyester content means it provides maximum moisture wicking. Castelli made it on the limit of not being too transparent in case you want to wear it for group training sessions, or if you're using it at home and want to take it off after warming up the long zipper means it comes off easy. Castelli put a couple of pockets on the back so you can stash your AirPods or phone or headband or whatever you haul to your training station. They've kept the collar low and the waist short for maximum cooling. It just happens to be a great hot weather jersey that you'll pull back out come August.