Please explain the parameter Overlay on button number 2

Please explain the parameter Overlay on button number 2.

A.
It means that this IP phone has more than one extension configured on line 2.

B.
It means any calls to line 1 when busy will be automatically forwarded to line 2.

C.
It means that the user can dial using any Overlay application, for example, Click to dial application.

D.
It means that the extension on line 2 is part of an Overlay group that handles calls that have been sitting in the queue and exceed the “Lay” limit.