When looking for an SD out micro SD card, the marking put on the card are there to help you choose the speed of card you need for the intended purpose. I found this chart that helps you determine which card speed you might need to record what your device is capable of.
Choosing too slow a card will result in data loss. Choosing too fast a card will work fine but cost you more if the device cannot perform the recording level that the card is capable of.
From the Adorama website:
The speed of the card you purchase is truly one of the most crucial aspects of buying an SD card, and is one that’s often overlooked by those unfamiliar with SD speed class ratings. It may seem complicated at first, but in reality, it’s fairly simple to understand.
SD cards are available six different speed classes, from slowest to fastest: Class 2, Class 4, Class 6, Class 10, U1, and finally U3. U1 and U3 cards are compatible with the UHS transfer bus used in newer devices, so you will often see a card rated with both U1 and Class 10. That means that when the card is used on a standard high-speed data bus, it will perform at Class 10 speed, but when used on a UHS bus, it will perform at U1 speed.
Class 2 – these cards are the slowest class currently available and are best suited for consumer-level point-and-shoot cameras, audio recording, and low-resolution SD video recording.
Class 4 & 6 – the “middle of the road” in terms of speed, these cards will work well with still cameras shooting jpeg files, and some very light HD video recording at 720p.
Class 10 & U1 – these cards are capable of recording at up to 10 MB/s, so they are well suited for pro-level still cameras shooting RAW photos, and HD video recording at up to 1080p. This is a great speed to purchase if you’re looking for a balance between compatibility with your current equipment and future usage as well.
U3 – these cards are overkill for most uses, but are absolutely crucial if you’re shooting 4K video with your device. 4K video files are large, and require a card with a high sustainable speed to be able to write them properly. U3 cards are quite a bit more expensive than the slower speeds, but if you need them, you really shouldn’t risk using anything slower.