The question Why are there advertisements in the Russian ISS flight control room? shows the image below. There is a large red boundary that looks like the boundary of the overlap of several circles drawn on the surface of the Earth then distorted by the map projection. I'm guessing that each circle is the limit of the range of something, that is most likely a ground link for telecommunications. I can not read Russian but I do see TDRSS mentioned at the bottom.
That the boundary appears so prominently suggests that it has some specific meaning, and makes me wonder what happens differently when the ISS is inside it compared to when it is outside of it.
Question: What exactly is this boundary, and what happens when the ISS is inside it?
cropped and sharpened:
Source: Russian ISS Flight Control Room (click for full size)
View of the Flight Control Room at Russia’s Federal Space Agency Mission Control Center in Korolev, Russia, located on the outskirts of Moscow.
Date: 21 April 2004
Author: NASA/Bill Ingalls