That is a designation applied to SDHC cards only.
It just means it can read/write faster.
It will be of interest to people who use DSLR (digital single lens reflex) cameras.
Your camera (SD500) will not benefit from having a faster memory card.
Here is a link....scroll down until you see the paragraph called SDHC:
Compact Flash cards have capacities all the way up to and including 64 gig.
Your S400 camera would be limited to 2 gig.
Before buying a card larger than 512mb you should check with Canon and see if it can handle 1 gig or 2 gig.
Come cameras can and some cameras can not.
At the time you camera was made, there were no cards larger than 512mb.
Joe, or anyone else. Please explain the difference with these SDHC Class 4 or 6 memory cards. Will I be able to use one with my older Canon SD500 camera?
I like the camera a lot. Great pocket camera.
Looks like I am limited to an SD 2.0GB memory card? I have two of them for travel pics. I would like a higher memory SD card.
Same kind of question for my other Canon S400 pocket camera. Right now the card has a 512 Flash Card. Anything higher available?