One set contains 2 different sets of petals with different grades of stiffness. In principal it can be said that the stiffer the reed, the higher the engine will rev without the reeds flapping about uselessly. The thin reed (softer than the thick ones) gives a better engine pick-up from low revs though. An additional advantage of the fibre reeds is that they, unlike their steel counterparts, are less likely to ruin your cylinder in the case of a failure.