That's the section that goes from the primary cat and would have a race cat on it, or no cat at all.
The Aussie works like this: DP, eliminates 1st cat, midpipe, 1st cat through 2nd cat, and the third section is a 2nd cat back piece.
I went to a shop to have a full 3" done. They said theycouldn't do a down pipe and they just used mandrel bends they purchased and welded into the system. They also said they couldn't fit a 3" piece over the rear axle, so they went under.
In the 5 years I've been part of this comunity, the only place I've seen anyone get a full 3" mandrel bent custom system is Australia. It was only after a few years of us Americans whining the Aussie stepped up and made a mass produced system, and they're the only one's who have done it.