What's the difference between Extreme SDHC and Extreme HD Video SDHC?

Extreme SDHC Card: SDSDX3-016G-A31 Extreme HD Video SDHC Card: SDSDRX3-016G-A21 Both are class 10, 30MB/s It’s odd that the market is flood with SDSDRX3-016G-A21 (class 10, 30MB/s), but the sandisk website shows it’s class 6, 20MB/s. More strange than that, there is noooooo SDSDRX3-016G-A21 (class 10, 30MB/s) sold by sandisk’s own online store!!! I talked to 2 agents from sandisk online support, for the same device I have, one suggests me to buy SDSDX3-016G-A31, while the other one suggests me to buy SDSDRX3-016G-A21. agent: Technically both the cards are same. me: but why they have different names and mfr#? agent: This SDSDRX3-8192-A21 is manufactured for recording videos, however you can still save pictures and other data as well… me: so they are indeed different, SDSDRX3-8192-A21 is better for HD video as the name indicates? agent: Functionality for both the card will be same however the only difference is in the name. I am so confused by those statements, there is no clear answer on whether they are the same or not. It’s so weird that there is no piece of information mentioned about SDSDRX3-016G-A21 (class 10, 30MB/s) on sandisk’s website. I like sandisk’s product, but such inconsistent information really scares me away.

Hmm, for this mysterious SDSDRX3-016G-A21(class 10, 30MB/s), I saw two different versions from authorized resellers, one has “SDHC” label, the other one has “SDHC I” label. It seems the one with “SDHC I” supports UHS-I, but it’s NOT extreme PRO (UHS-I, 45MB/s), although extreme PRO also has “SDHC I” label on it. This is really a big mess for an established brand.