According to the latest official specifications from the SD Association (SDA), the speed labeling rules for SD and microSD cards have been officially unified. The C, U, V, and E symbols on cards coexist to indicate minimum sustained write speeds and video classes; they do not represent storage capacity.
Speed Class is the oldest system, symbolized by a 'C' inside a circle. It has four tiers: C2, C4, C6, and C10, with minimum sustained write speeds of 2, 4, 6, and 10MB/s respectively. While less common on new cards today, some products still use this classification.
UHS Speed Class is denoted by the letter 'U' and has only two tiers: U1 with a minimum of 10MB/s and U3 with a minimum of 30MB/s. U1 is found only on cards with UHS-I interfaces, while U3 can be found on both UHS-I and UHS-II cards. For 4K video recording, U3 is basically the entry-level requirement.
Video Speed Class is written as 'V' followed by a number, where the number represents the minimum sustained write speed, ranging from V6 to V90 (covering 6 to 90MB/s). Higher classes provide more headroom for high-bitrate video. 8K video requires V90 level, while high-bitrate 4K needs at least V60.
The fourth set, SD Express Speed Class, uses the symbol 'E' and appears with SD Express interface products. It is divided into four tiers: E150, E300, E450, and E600, with minimum write speeds of 150, 300, 450, and 600MB/s respectively. This is the highest-threshold set among the four classification systems.
It's important to distinguish that C, U, V, and E classes govern the minimum write speed, while the bus interface determines the maximum transfer speed. UHS-I has a theoretical maximum of 104MB/s, UHS-II reaches 312MB/s, and UHS-III hits 624MB/s. If you insert a UHS-II card into a UHS-I slot, performance will be capped at 104MB/s, meaning even a V90 rating on the card won't be fully realized.
The most common misinterpretation involves C10 and U1. Their definitions are identical, with a minimum write speed of 10MB/s for both. However, many manufacturers still print both symbols on their cards. This isn't to indicate added performance, but rather to accommodate different users' reading habits.
The card selection order is actually quite clear: first, check which bus generation the device uses, then choose the U or V class based on the recording format, and finally consider capacity. The speed class is a minimum guarantee, not a peak performance claim.