I have seen postings where people say that they have exhaust from a 99 or 00 Si. What do they mean by this? Do they just mount the muffler to their exhaust system?

 

It means exactly as they said it...replaced their exhaust with the ones from either a 1999 or 2000 model Si's. They probably have a DX/EX/HF model and needed an upgrade. Although I'm not sure what exactly the difference is in each model. Probably diameter...