So far as I know it is an agreed upon limit such that when the Sun reaches this position below the horizon, 'astronomical twilight' ends. I believe this means that stars at the naked eye limit can just be seen for the first time at aa apparent visual magnitude of +6. I do not know this for absolute certain and will try to check on this later. Civil Twilight ends when the center of the Sun is 6 degrees below the horizon where the brightest stars are just visible. Nautical Twilight ends when the Sun is 12 degrees below the horizon and you can no longer see the horizon at sea.