Sandisk SD Card Ultra/Extreme/Extreme Pro for RX100?

Pardon my ignorance, but is the speed rating for upload or download, or both?

Is speed the only difference between the Ultra, Extreme and ExtremePro?

And, while I'm at it, what does the class refer to?  I can only see class 4 and class 10 (i.e. nothing in between), so it doesn't appear (to me) to be a signifier of incremental performance.

Sorry for jumping on the thread to satisfy my curiosity . . .

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Cheers,
Damien