USB Restricted Mode brings a little extra security to your iPhone or iPad.
Here's how you enable it.
While Apple has been testing it since the later beta versions of iOS 11.4, with the release of iOS 11.4.1, USB Restricted Mode is now available to the iPhone-using public at large. This new mode, which is buried under your passcode settings, adds additional security to your iOS device by preventing USB accessories to connect with your iPhone or iPad if the device has been locked for more than one hour.
Tap Face ID & Passcode or Touch ID & Passcode depending on your device.
Tap the switch next to USB Accessories under Allow Access When Locked to put it in the white "off" position (if it's not there already) if you want USB Restricted Mode activated. If it's in the green "on" position, USB devices will have the same level of access that they've previously had.
That's all there is to it. While the companies behind devices like the GrayBox have previously said that they can beat this new USB Restricted Mode, we'll have to wait and see whether or not that's true and what's more, what Apple will do to respond.