Rosin comes in light and dark varieties. At D'Addario, the light rosin is hard and the dark rosin is soft. The main difference between light and dark rosin is the melting point, which means it's more or less sticky.

Light rosin (hard):
Is less sticky, making it easier for people with lighter bows or who may not need the extra leverage on their bow. This is good for players with higher strings, such as violins or violas.

Dark rosin (soft):
Has a lower melting point and feels stickier. The extra stickiness helps players grip the string with the bow hair, making it ideal for those playing instruments with wider/longer bow hairs (e.g., cello or viola).