"Question: What do these S[number] things mean?"

#PokemonGO: So I keep saying these posts all over the place where people mention these S2 or S17 or S16 or whatever number things, and I don't know what this means. I looked at the posts with them and it doesn't make sense, all I can understand is that it might have to do with a bunch of stuff in the game, like gyms and biomes. But I don't really understand what it is. Could somebody tell me what it means? via /r/TheSilphRoad http://ift.tt/2FgGZDx