What is the U1/U3 speed class?

Some of the new UHS memory cards may have a U1 marking, which refers to the U1 Speed Class designed for UHS-I card. The U1 Speed Class designates a minimum capture (write) performance of 10Mbytes per second for recording streaming video content. Note that although U1 is the same speed as Speed Class 10, they are tested differently, and meeting one does not imply that the other is met by the card.
U3 Speed Class means that the memory card performance needs to reach 30MB/s to meet 4K resolution recording and play.