(
AFAIK- and please correct me if wrong)
1). 'Organic' brake pads (or shoes in this case) are softer and will conform to any surface..but will wear out faster and with more dust.
Look at any BMW rim.
2). 'Semi-metallic' brake pads are harder and less prone to overheating..but will wear out a disk or drum surface faster and may have less stopping power.