The Steam Deck is spec’ed with a UHS-1/SDHC slot, which means that you can’t use SDUC-class cards and you won’t get much benefit from using comparable cards with a UHS-II/UHS-III bus mark compared to one with a UHS-I mark, even if the other marks otherwise suggest better performance. You can basically ignore the A/V markings because they’re not granular enough to help with comparing cards at this particular performance level (you should instead compare “Random Read”/“Random Write” performance benchmark scores).

Note that there remains a considerable amount of variance among similarly marked cards. For example, the Sandisk Extreme Pro (Bus: UHS-I, Speed: 3) can benchmark write speeds which are almost twice as fast as the Sandisk Extreme (Bus: UHS-I, Speed: 3).

tl;dr: The ideal card will have the following markings:

• Capacity Standard: SDXC (SDUC is not compatible)
• UHS Bus Speed: I (higher is fine, but not helpful)
• Speed Class: 3 (though you should really be comparing benchmark scores instead!)

I bought an “U3 V30 A2” SD card and it works great. I might be wrong, but I think I’ve heard that it’s useless to go anything faster.

Also, the most important thing: buy from a reputable source. There are A LOT of fake SD cards out there.