To take a group portrait, for example, you might use an accessory flash bounced off the ceiling as your main light, and use the built-in flash as a frontal fill light to open up shadows in faces. Dedicated electronic flash units provide the most versatile flash available. A dedicated flash unit works only with specific cameras. Contacts in the flash foot complete circuits with identical contacts in the camera hot shoe. Other cameras set the dedicated mode automatically when you attach the flash to the camera.
Most dedicated equipment is made by camera manufacturers for their own cameras.
Some universal dedicated flash units can be adapted to a variety of cameras with accessory modules, but most offer fewer features than a same brand flash. Read your camera manual and talk to your photo dealer before attaching any dedicated flash other than one designated to be used with your camera. Though they can handle complex tasks, dedicated flash units are extremely simple to use, because they'll make all the exposure decisions for you. Once you attach a dedicated flash to your camera, you are ready to start taking flash pictures. No calculations or complex tables are required.