Just as you can make a standard exposure brighter or darker using the exposure comp setting, you’re able to use flash exposure comp to increase or decrease the
brightness of the subject lit by the flash. E-TTL II flash metering is intelligent, but there’s still a chance that it can be thrown by reflective objects such as mirrors and windows, for example, causing it to make the flash exposure less bright. Or you may just want to reduce the flash exposure so that you only have a subtle amount of fill-flash to lift the shadows in a daylight photo.
You can use up to +/-3 stops of flash exposure comp, or combine it with flash exposure bracketing for more substantial adjustments