What is LD5 on the PCA10028 used for?

[SørenHN]

When plugging the PCA10028 into my PC, LD5 is permanently turned on (or blinking to fast to see it). But if plug the board into at power outlet, LD5 starts blinking rapidly. I have noticed the LED is blinking while flashing the board, but what is it actual used for?

[MartinBL]

LED5 is connected to the Segger MCU and is only used to give an indication of activity between the MCU and whatever device you have plugged the other end of the USB cable into. It is not directly used by the nRF51 at all and I'm actually not sure what the various blinking patterns mean. The firmware running on the MCU is not made by Nordic.