For the latest and greatest action cams or even cellphones like the Note 4, microSDXC cards are essential. With 4K video output becoming a norm and high bit rates following it, flash memory with high write speeds is in great demand. Typical UHS-I U1 cards may not be able to keep up, that is where UHS-I U3 cards come into play. The U3 designates a minimum write speed of 30MB/s, plenty for most 4K video recording. High read speeds also allow for faster file transfer time from the card to PC. The faster you can copy all your media the faster you can get to work and share.