The Visibility Zone is calculated around a transport's initial planned route (see here for more information on the initial route). It acts as a buffer that controls vehicle visibility on the map—showing vehicles inside the zone and hiding those outside it.
When GPS data indicates a vehicle has moved outside this zone, its position becomes hidden.
If tracking fails, you'll see a specific Tracking Cancelled reason (see here for more details) that may indicate this situation.
Note: GPS data sent from an in-house system to Visibility is fully trusted and not subject to Visibility Zone restrictions.