Category is used to describe how much light is filtered out.Category 0: Clear lensCategory 2: Lens with a lighter tint.  Great for cloudy, dull conditions, late afternoon. Preferred when in and out of bush (dark to light); fast speeds demanding optimum definition; those who do not want dark coverage, or are not used to wearing sunglasses.Category 3: Lens with a darker tint.  Sensitive eyes, full sun.