When Secure Digital (SD) cards were first introduced, their main purpose was as a form of storage to stream data from. However, how that storage is used has changed dramatically in recent years, with as much importance placed on fast and repeated reading and writing to the cards. This has led to the introduction of a new classification system being introduced by the SD Association.
SD Specification 5.1 introduces Application Performance Classes for cards, which ensure a minimum read and write speed for Input/Output Operations per Second (IOPS). Until now, SD cards have carried a speed class rating ensuring a minimum sequential write speed. For example, a Class 2 card guarantees a minimum of 2MB/s sequential writes, where as a Class 10 card increases that to 10MB/s. The higher speed is required for common operations such as recording HD video.
A guaranteed sequential write speed does not suffice for running applications, though, which has become a common task of SD cards used in smartphones or storage-limited cheap laptops. For that, guaranteed random read and write speeds are also useful.
As AnandTech reports, This is where the Application Performance Classes come in, the first of which is A1 (short for Application Performance Class 1). An SD card carrying the A1 or A1 App Performance logo will guarantee 10MB/s sequential read performance just like a Class 10 card. However, it also guarantees 1,500 read IOPS and 500 write IOPS performance. If you are purchasing an SD card to install and run software on it, an A1 certified card will provide the best available performance.
For now, A1 is the only Application Performance Class available, but the SD Association says higher classes will be introduced when they see a need. Another important thing to note is that A1 SD cards will only produce their guaranteed performance when used with SD Specification 5.1 hardware. With older hardware, you may not enjoy the full benefit of an A1 card.