Fan speed is typically related to the different modes or settings that enable you to control the air purifier. Cleaning levels are part of the settings.
The cleaning levels are optimized to capture specific airborne particle sizes. For example, some particles are smaller and are more effectively captured in the filter at lower fan speeds. Sleep settings also run the fan at lower speeds to keep noise levels to a minimum. Allergens are larger airborne particles and fan speeds are higher on this setting to capture allergens in the air more quickly on this setting.