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Definition: SD card classes

SD card classes designate the minimum write speed of the flash memory card for recording video. The three class types are Speed, UHS Speed and Video Speed. Devices that support any of the UHS buses also support the earlier Normal and High-speed buses. See SD Card and SD Express.
        Bus Type    Write   Highest
                    MB/sec  Recording
 Speed Class
  C6    Normal         6      HD
  C10   High-speed    10      HD

 UHS Speed Class (2010)
  U1    UHS-I/II/III  10      HD
  U3    UHS-I/II/III  30      4K

 Video Speed Class (2016)
  V6    UHS-I/II/III   6      HD
  V10   UHS-I/II/III  10      4K
  V30   UHS-I/II/III  30      4K
  V60   UHS-II/III    60      8K
  V90   UHS-II/III    90      8K

        Bus Type    MB/sec
        Normal       25
        UHS I       104
        UHS II      312
        UHS III     624
        SD Express  985

Speed Class Markers
The C, U and V designate the speed class, while the Roman numerals are the bus type. (Images courtesy of SD Association and SanDisk Corporation.)

More Pins
Starting with the UHS-II bus, the cards have a second row of pins. (Image courtesy of SD Association, www.sdcard.org)