USB Restricted Mode for iPhone
iPhones and iPads have a USB restricted mode which prevents USB accessories plugged in through the lightning port from accessing data on the Apple devices, if they have been locked for longer than 60 minutes. The devices can still charge, but will not be able to transmit data. The protective measure makes it harder to crack the passcode for these Apple devices, since some forms of software can prevent an iOS system from locking after several failed passcode attempts by connecting through the lightning port.
USB Restricted Mode can be accessed under iPhone under Settings . . . Face ID & Passcode . . . .USB Accessories.
. . . when the option for 'USB Accessories' is not selected, the iPhone will be in restricted mode, as it is by default.
Note that USB Restricted Mode can sometimes prevent your phone from charging at all, so toggle this option if you're having trouble powering up.
USB Restricted Mode can be a significant hurdle in mobile electronic discovery. Cellebrite offers a USB Restricted Mode bypass feature, and GrayShift's GrayKey can get around this limitation as well, but it may still be a bar to collecting data from cell phones in some cases.