There is a possible typo in the nRF5340 DK documentation.
https://docs.nordicsemi.com/bundle/ug_nrf5340_dk/page/UG/dk/ext_programming_support_P20.html
In the top picture, on P20, the pin in between RESET and VIN 3-5V is labeled "GND DETECT" and is wired to the custom board.
HOWEVER, in the very next image, this pin is designated as "N.C."
Which one is correct? and does that mean the diagram for hooking up a custom board needs to be tweaked?