With AI Servo using the full 19 focus points, you can select a single focus point that the camera will begin tracking focus with. As the subject moves through the frame the other focus points will pick up the subject as it moves behind them. For example, if you are tracking an actor on a stage who moves from one side to the other and you always want to keep the whole stage framed - then this would be a good approach.
If you can pan the camera to keep the subject in the same area of the frame, then it is better to use a single point only, or an expanded single point. For example if you were tracking a bird in flight.
If you use Zone AF with AI servo, then you can't select a starting focus point, so the camera will pick the closest subject in the zone, just as it would with single shot. However, if the chosen subject moves within the zone, then the other focus points will engage when the subject moves behind them.
If a subject moves outside of the focus point(s) you have activated (i.e. outside the zone you have selected) then the camera cannot focus on it further.