P12: Reserved For Accessbility
P12 is a loosely specified pin for Accessibility purposes on the micro:bit. It was intended to be a UART Transmit pin that would stream accessibility data to an accessory, or perhaps a pin to enable/disable an on-board accessibility mode over USB.
Nevertheless, if you're creating an accessory you should not attempt to use this pin in your design. If you're just tinkering you may use it for now- but don't rely on it being available in your language/editor of choice!
For more information and discussion, see: https://github.com/lancaster-university/microbit-dal/issues/256
And additionally: http://tech.microbit.org/hardware/edgeconnector_ds/