That header passes through the high-voltage 48V PoE supply from the Ethernet port so that it can be stepped-down or regulated by 5V PoE hat or voltage regulator. This regulated 5V supply can then be passed to the 5V input pins on the 40-pin header.
An example PoE hat that can do this automatically is here: Buy a PoE HAT – Raspberry Pi
It will take the 48V high-voltage from the header, step it down to 5V, and pass it back to the 40-pin header. It can be mounted on top of the Nano devkit. Or you can do this manually by wiring in your own 48V->5V regulator.
I can’t seem to find a pinout of j38 - there is nothing in the manual. Can anyone help?
Also, the manual says that POE should be IEEE 802.3af. What would happen if i plug in IEEE 802.3at (POE+)?