The watch operates using cellular signal via cellular towers: it does not require Wi-Fi or Bluetooth. The watch has full functionality everywhere in the United States where it can establish a cellular connection. For maximum benefit, you should wear the watch at all times (including sleeping), and remove only for limited instances (like bathing or swimming).
If you are out of cellular network - say, traveling abroad - the watch's heart rate sensor will continue to track heart rate and activity. But in the event of an emergency, no cellular or emergency communication will be available.