Window performance standards define window products by Performance Class (residential, light commercial, commercial, heavy commercial, and architectural) and Performance Grade (15, 20, 25, etc.). Local building codes likely define the minimum Performance Grade. In general, R and LC class windows have smaller test sizes and are allowed less force to operate and lock. But in many cases they offer sizes, design wind pressures, water resistance, air leakage resistance, forced entry resistance, and hardware performance associated with the C, HC, and AW classes.
The Performance Grade is based on the design wind pressure, calculated from the building code, as noted previously. If the maximum design wind pressure is 25 pounds per square foot, then the Performance Grade needs to be 25 or greater. Performance Grade is designated by a number following the product type and class designation.