The “Class” rating is the guaranteed minimum write speed in megabits/second. It has no affect on playback. It’s really only important on devices like digital cameras thatneed to save large amounts of data quickly.
“HC” stands for High Capacity. Cards below 2 gigabytes are just microSD. Cards from 4GB to 32 GB are microSDHC. All newer (the last 4-5 years, if not longer) expandable Sansa players, the Fuze included, are compatible with all microSD and microSDHC capacities. Cards larger than 32GB are microSDXC (extended/expanded capacity). These are NOT currently compatible with any Sansa players, but few OTHER devices accept them either.