UHS speed Class 3 has been announced, which is a new high performance class that supports 4K video in its 4096 × 2160 (8.8-megapixel) resolution. The new class will be available for SDXC UHS-I and UHS-II and SDHC UHS-I and UHS-II memory cards and devices.
UHS speed class 3 defines a minimum of 30 Megabytes per second for high quality video and this should be enough even for large 4K video format files. The SD Association came up with UHS-I and UHS-II high performance bus interface that can support data transfer between SD memory card and device or up to 104 megabytes for UHS-I and 312 megabytes per second for UHS-II as a theoretical limit. Hoover, the actual speed will be much slower, but effective enough.
4K videos, live broadcasts and content can be recorded on high-performance devices such as Digital Single Lens Reflex (D-SLR) cameras, Digital Single Lens Mirrorless (D-SLM) cameras, camcorders and video cameras and then played back smoothly. New devices offering the UHS Speed Class 3 will be backwards compatible and will work with existing SD memory cards.
The compatibility of UHS Speed class 3 cards with existing devices, but at lower speeds, is unsure at the moment. There has been a lot of talks about the SDXC cards working on some unsupported devices, however the odds are that they will not work unless specifically stated. More about speed/benefits can be found here and information about compatibility can be found here.