You are correct It refers to the guage on a double butted spoke. 2.0mm at both ends where the brute strength of larger diameter is important to reduce fatigue factor, and 1.8mm center for 'better springing characteristics.... (which) relieve the spoke elbow and thread ..., when under excessive load..." (Schraner p 18). I recommend that you read 'The Art of Wheelbuilding' by Gerd Schraner especially Part 2 on Wheel Components. Very helpful source of info.