It's both... technically.

After the rear header, it collect to a 3" pipe, hits the turn still 3", connects to a "race pipe" (where CAT goes), then to a 3"/2.5" adapter that bolts to the rest of the exhaust system. If you run a full 3" you start at the race pipe or converter and go out from there. You just have to weld in a bung for the rear 02 as it sits in that rear adapter.