How does 200x or 300x in relation to an SD card differ from the class rating

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I've always understood "class 10" (or whatever) to refer to the speed of an SD (or SDHC) card, but apparently ratings like 200x or 300x also refer to speed. How do the two rating systems differ? Which is more standard and more reliable? Is something like 300x simply used because there's no such thing as a class above 10?

Best Answer

The speed class rating is the SD Association standard. These numbers are the ones you see that range from 1 to 10 and will be inside a U or a C. They represent the card's minimum performance. The "x" rating is a commercial rating. Manufacturers will give this number and it is the maximum potential of the card.

Lexar has a (PDF) white paper on the whole thing titled Understanding SD Association Speed Ratings.  Additional information is available at Making Sense of Memory Cards.