Asus P8P67 WS Revolution - Manual (Page 40)

3. EPU LED
The EPU LED lights when the EPU switch is turned to Enable.
4. Standby power LED
The motherboard comes with a standby power LED that lights up to indicate
that the system is ON, in sleep mode, or in soft-off mode. This is a reminder
that you should shut down the system and unplug the power cable before
removing or plugging in any motherboard component. The illustration below
shows the location of the onboard LED.
2-14 Chapter 2: Hardware information