1. Zoom. This controls the spread of light from the flash. The numbers correspond to lens focal length, so 24mm is a wide beam spread, and 105mm is a tight spread. To see this in action, stand a few feet from a wall and shoot the flash at 1/64 power using different zoom settings. See the difference in how wide the flash pattern is.
2. Power. Manual is manual -- the flash *always* fires at the same power output every time. You have to set the appropriate aperture (and shutter speed) manually on the camera. You set the power level from full (1/1) down to 1/128 in 1/3's of a stop.
This is useful when you are using the flash off camera, possibly on a light stand with an umbrella. You set the appropriate flash power and manual camera settings, and the flash puts out the same amount of light every time.
You may be thinking of the original type of "Auto" flash, in which you set the flash and the camera to a particular aperture setting, and the flash varies the output to match the camera-to-subject distance. If you're going to do this, you might as well use the TTL system, setting the camera to your chosen aperture. The flash will vary the output to match your aperture (within limits).
