Nikon Speedlight SB-800 review and tips - non-TTL auto flash modes
Non-TTL auto flash (A)
As the name suggests flash output is controlled automatically but not by the camera - instead, it is managed only by the speedlight. There is a light sensor located on the front side of the flash - the speedlight analyses reflected light that it receives from the subject and determines flash output on the basis of the aperture, ISO (needs to be set in Custom Settings) and focal length of the lens that one inputs manually.
Note that if you use the speedlight with a medium format camera you should think in terms of 35mm format when adjusting zoom head position - for instance, if you use a standard lens (80mm) you should set zoom head to 50mm.
As to the shutter speed, you can choose any speed provided it is slower than your camera's top sync speed. The difference is going to be in the amount of ambient light captured - the slower the speed the greater the impact that ambient light is going to have on the image. In flash-only photographs shutter speed is entirely insignificant. If you would like to achieve a fill-flash effect outdoors you should first determine exposure for background/ambient light and then set respective aperture on the speedlight.
Note that in this mode flash output compensation cannot be set on the speedlight and you need to tweak aperture. For example, if the aperture set on the lens is f/5.6 and you wish to make a -1EV compensation you then simply have to set f/4 on the speedlight.
Non-TTL auto aperture flash (AA)
The difference between this mode and the one above is that all information (i.e. ISO, aperture and focal length of the lens) is fed to the speedlight automatically when the flash is used with CPU lenses and cameras that support this mode; another dissimilarity is that you can set flash output compensation directly on the speedlight by pressing SEL and then "+" or "-" buttons (this is the only parameter that you can change on the flash). When the flash is mounted on a camera which supports this mode with a CPU lens the AA mode is automatically selected over the auto flash mode (A). If need be this can be changed in Custom Settings, though.
Non-TTL auto flash mode is quite useful when using the speedlight with manual cameras. When the flash is used with modern cameras and lenses, though, I cannot think of a situation where I would prefer using the modes described here over more sophisticated camera-controlled TTL modes.