The SDHC/SDXC cards gradually supplant CompactFlash cards limited by parrallel interface and fragility. The SD standard (Secure Digital) has evolved over time to offer more speed and increasing capacity. Four major revisions relate mainly to the interface bus whose last version are UHS-I (type I) and UHS-II (type II). SD cards use a connection to a serial port via sliding contacts less fragile than the 50 pins connection of the CompactFlash card.
To characterize SD card speeds, manufacturers rely on a 3-stage classification: speed classes C2, C4, C6, C10 allow a minimum sequential writing from 2 to 10 MB/s. UHS speed class (Ultra High Speed) U1, U3 allow a minimum sequential writing of 10MB/s (as class C10) to 30 MB/s with an UHS bus type I or type II. Finally V6 video speed classes, V10, V30, V60, V90, defined for video media, allow a 6 MB/s sequential writing (as the C6 class), 10 MB/s (as class C10), 30 MB/s (such as C30 class) for 4K video, up to 90 MB/s provided for the 8K video recording. So, the same card can be C10/U1/V10 or U3/V30. A U3 card is not necessarily with a UHS-II bus!
Type SD UHS-I cards have only one line of 8 contacts on the edge. Type SD UHS-II cards (provided primarily for video) have a line of 8 additional contacts on the middle.
Three sizes formats are available: the original SD format, the most used in the audio/video professional world, miniSD format (half height, little used) and microSD format commonly used in smartphones.
Suffice to say that the sound mixer can be confused in choosing the right media for his or her recording mixer!
The only solution is to conscientiously respect manufacturers compatibility lists who usually rely on Sandisk brand. DC Audiovivuel is able to provide SDHC or SDXC cards that comply with the recommendations of the recorders and mixers manufacturers.
Although the connection of the SD card is via sliding contacts, you should never force it, at the risk of blocking it in the slot. If it does not fit is that it is the wrong way!