Was wondering what the big bolt on the bottom of the mufler is for Thanks

 

It's a drain bolt in case you sink the thing.

 

On my son's 300ex, the stock muffler had a small plate on the bottom, the owners manual said it was to blow out any accumulated rust or carbon,you put a rag over the end of the muffler while it was running,rev it up a few times to see what comes out,and put the plate back on. Perhaps that is another function of the bolt on the Polaris.