Indicator light on thermostat of a hydronic heating system

I noticed this indicator light in my thermostat display (marked with a red oval in the picture). I have a hydronic heating system that I have been using for the past 7 months. I never saw this before. Any idea what it is for? I couldn't find any documentation on the thermostat manufacturer's website. This indicator light is always on whether the heater is on or off.

Thats Chinese for "Made in China"