* The existing Speed Class symbols for non-UHS SD, SDHC and SDXC products are Class 2, Class 4, Class 6 and Class 10. They refer to the minimum write speed performance of the memory card.

New specification creates more storage expansion options for mobile users to run applications from SD memory cards at optimal levels The new App Performance Class symbol will identify SD memory ...

The many symbols, numbers, and letters that cover an SD card are evidence of that. Each one represents a different standard that's improved since SD cards were invented.