What are the areas for champion masteries

I am a bronze two lee sin main (not great at the game but can honestly say I am VERY good at lee sin especially compared to people in bronze-high gold) and I recently got an S+ on a game where I went 17-6-26. I read on I think it was surrender@20 that an S+ means you are like the top 2% in your area. what are the areas that it is talking about?