The Raspberry Pi Revision 2.0 introduced a new header to the PCB. This header is called P5 and consists of 8 pins. It is located directly underneath the standard 26 way P1 GPIO header.
It was intended that this header is populated on the underside of the PCB so any connectors or cables do not interfere with the P1 header. Pin 1 has a square pad whereas the other 7 have circular pads.
The P5 pins provide :
- +5V, 3V3 and Ground
Here are some photos showing the location of the P5 header in relation to the 26 way P1 header :
This header is unpopulated to use it you need to solder your own pin header to the board. The intention is that you would solder a pin header to the underside of the board to avoid getting in the way of P1 and the other headers on the topside.
This is why P5 is labelled on the underside of the PCB but how you solder bits to the board is ultimately your decision based on your requirements.