The SD association's name system seemed to be mostly centered around marketing.
https://en.wikipedia.org/wiki/Secure_Digital
mmc What existed prior to 'SD'
https://en.wikipedia.org/wiki/MultiMediaCard
SD: <= 4 GB (Though true 4GB SD cards were/are rare, I have a few that I used with my TW Zodiac - they existed). Generally FAT formatted.
4 GB-32 GB SDHC Generally FAT32 formatted
8 GB - 512 GB SDXC (theoretical 2TB max). Generally fat formatted up to 32GB. Some 32GB and most larger cards come exFat formatted.
Now there are SDXC UHS-II cards that use a whole new pin layout - though the same physical card size.
https://www.sdcard.org/developers/overview/bus_speed/
The Pyra's SoC and thus it's SD card ports are UHS-I speed which means that they top out at 104MB/s theoretical. How close it will come to that in reality? No idea.
The new UHS-II cards with the additional row of pins could theoretically reach 312MB/s, but good luck finding something to plug them into that can keep up with that at this stage.
In all of this, though, keep in mind that the stated MB/s rating on most cards is 'burst' only and they're generally unable to sustain those speeds. That's why it would be good to have a program that benchmarks cards and can load them up to a centrally maintained database (with permission) so that we can quickly build a listing of how cards perform.