There are a number of different markings that may or may not be found on any given SD or microSD card. The markings help the user determine which purpose the card is best suited for among other information including:

• Memory capacity
• Minimum read and write speed
• Various read and write speed information

Memory Capacity

Most cards will display the memory capacity as a number on the card prominently, typically as Gigabytes (GB). However, they can be in Megabytes (MB), though it is not as common. The below example card is 64 GB. 

Format

There are four types of memory markings that represent the memory range or "format" for a card (in most cases):

• SD or SDSC (Standard Capacity) which means the card can have a maximum of 2GB (but could be less)
• SDHC (High Capacity) - the card ranges from 2 to 32 GB
• SDXC (Extended Capacity) - card ranges from 32 GB to 2 Terabytes
• SDUC (Ultra Capacity) - card ranges from 2 to 128 TB (not common)

Speed Classes

There are 3 main different types of speed classes. The SD association created this speed classification system to help determine what cards are best suited for various purposes. They all represent the same metric of a given card which is the minimum read and write speed of a card in MB/s (Megabytes per second). A card can have 1 or more of these markings but typically has no more than 1.

1. Speed Class

One marking you may see on an SD card is the speed class marking. This class is at the lower end of the speed spectrum. This marking usually comes in the form of a number in a circle (as shown below) and represents the minimum read and write speed of the card in MB/s (Megabytes per second). This class can come as a 2, 4, 6, or 10. 

A class 6 card can generally be used to film in HD and shoot basic JPEGs.

A class 10 card can be used for burst shooting, full HD, some 4K filming, and raw photography.

2. UHS (Ultra High) Speed Class

The next speed class up is called UHS. Again, this symbol represents the minimum read and write speed of a card. It is printed with a number inside of a U and has two variants:

• Class 1: 10 MB/s (equivalent to speed class 10)
• Class 3: 30 MB/s

Many 4K cameras won’t accept SDs without a UHS class 3 rating. The UHS class 3 rating is generally a minimum requirement for ultra-high resolution filmmaking.

3. Video Speed Class

The last and highest reaching speed class is called the video speed class. 

This class has the following variants:

• V6: 6 MB/s
• V10: 10 MB/s
• V30: 30 MB/s
• V60: 60 MB/s
• V90: 90 MB/s

V60 and V90 speeds are often needed for filming in 8K.

Bus Speed Class

Another speed class that is helpful to know (but not always neccesary to know) is the Bus Speed Class. Each bus speed category is generally determined by the peak bus throughput. This is the maximum amount of bytes per second that can be exchanged between the host device and the SD card. Categories include:

• Default speed (DS): 12.5 MB/s
• High Speed (HS): 25 MB/s
• Ultra-high speeds:
  • Ultra-High Speed I (UHS-I): 50-104 MB/s
  • Ultra-High Speed II (UHS-II): 156 - 312 MB/s
  • Ultra-High Speed III (UHS-III): 213 - 624 MB/s
• SD Express:
  • 985 - 1969 MB/s
  • 1969 - 3964 MB/s

Only one set of contacts are used to send and receive information in UHS-I, so the bus will run in what is called Half Duplex mode when at a higher speed. This means at any given time, the SD card will only be able to send or receive data.

UHS-II and III contain extra contacts, which allows Full Duplex (send and receive together), to both occur simultaneously. Though, in UHS-II this results in the bus being made to run at a slower speed. UHS-III and SD Express don't have this problem and always run Full Duplex.

Random Read Performance

An important key to measuring consistent system performance is the ability to handle lots of random data instructions (measured in IOPs, input/output operations per second). In 2015 the SD Association created two further standards: Application Class A1 and A2. Cards with this rating will have an "A1" or an "A2" visibly printed.

• A1
  • Random read performance: 1500 IOPs
  • Random write performance: 500 IOPs
•  A2
  • Random read performance: 4000 IOPs
  • Random write performance: 2000 IOPs

Very distinct hardware support is required for A2-rated cards. Lastly, a card that is rated A1 or A2 also entails that the write and read speed also has a video speed rating of at least V10 (10 MB/s minimum read and write speed).