High bay lights are normally used for ceiling more than 20 feet, low bay lights are normally used for ceiling lower than 20 feet, anyway, this is a simple way to distinguish high bay and low bay, the exact ceiling height will be depended on the requested and actual lumens on the lighting areas. Moreover, high bay light have a built-in deep reflector to distribute the light down from a high ceiling, low bay light have normal reflector and lens to distribute the light down from a low ceiling.