When any Ethernet port is running, this indicator will flash, and the flashing frequency is different from that Ethernet port.
It’s not documented and not available either in /system/LEDs.
My guess is bridge activity (so also covering wireless if that uses bridge) but I could be wrong.
Most AC3’s I have installed are in a cabinet so I don’t really care 
When the bridge only has wireless, this indicator does not light up
The latest HAP-ax3 also has this indicator
It’s really confusing 
