Service lights

[Schnuffi]

Hi I have just ordered the Lumia 640 yesterday and while going over the specs at a comparison site it mentions " Service lights " I did a search on this site but found no mention of this what is it ?

http://www.phonearena.com/phones/compare/Microsoft-Lumia-640,Nokia-Lumia-735/phones/9253,8891

 

[Guytronic]

I'm guessing here.
Might be a built in notification light for certain product code versions.

 

[Zulfigar]

Is PhoneArena the only site that mentions the service light? Because I'm not sure it has one.