There are four kinds of Speed Classes, “Speed Class”, “UHS Speed Class”,  “Video Speed Class” and “SD Express Speed Class.”

As a characteristic of flash memory, actual transfer speed varies. Variable speeds are difficult to reliably record streaming content such as video because it requires a constant writing speed. All speed class types assures minimum sequential card access performance including multiple stream access (in SD Express Speed Classes) under defined conditions specified by the standard. It is most important for video recording and other applications.  It is also applicable for any write or read applications that requires an assured minimum constant sequential access performance. 

		SD Standard	SDHC Standard	SDXC Standard	SDUC Standard
Capacity		up to 2GB	more than 2GB up to 32GB	more than 32GB up to 2TB	more than 2TB up to 128TB
File System		FAT 12, 16	FAT 32	exFAT	exFAT
SD Logo
Form Factor Size	SD	32 x 24 x 2.1 mm
	microSD	11 x 15 x 1.0 mm
Speed Classes	NS mode	C2, C4, C6
	HS mode	C2, C4, C6, C10,V6, V10
	UHS-I mode	—	C2, C4, C6, C10U1, U3V6, V10, V30
	UHS-II & III mode	—	C4, C6, C10U1, U3V6, V10, V30, V60, V90
	SD Express mode	—		E150, E300, E450, E600