SanDisk Extreme Pro SDXC Memory Card 64 GB

: x stands for the number of MB/s of sustained write speed. 4 MB/s of sustained write speed. 10 MB/s of sustained write speed. New Bus system for UHS compatible cards, UHS-I over 10 MB/s of sustained write speed. High-definition (HD) camcorders require Class 4 or above. The Class Rating is only required for filming videos.