RCT (rain echo attenuated compensation techningue?) is something quite different. Bit of a 'specialised' mode for trying to see through heavy precip' etc.
Actually, I think this function is for something similar. When selected, it paints the area blue whenever there is total signal attenuation. The blue areas might be better described using by modifying the ancient charting phrase "Here (may) be dragons". What you know for sure is that the system hasn't a clue what's in the blue.
PM