In addition to introducing a message to let users know how Bluetooth and Wi-Fi work in Control Center, on the iPhone X, there's also a small UI change designed to make the Control Center gesture more obvious. Whenever you're on the Lock screen, there's a small bar underneath the icons on the right side of the device, letting you know there's more to see there. The bar is not displayed on the Home screen of an unlocked iPhone X, nor is it displayed within apps.