That's their Helmholtz chamber to cut at least some of the drone from having a 3" diameter system.
(I did not see KnockOff's post explaining this before I posted it.)
The problem for me is that it prevents me from tucking the large single (Ultimate Racing) turn down tip I am using up further into the valence cut out, like can be done with the CP-E tail pipes/tips.
Mitch (CP-E's FiST R&D dude) is supposedly working on a rear 'box' muffler tail pipe section (with an INTERNAL Helmholtz chamber incorporated into it, just like Thermal does) to replace that 'branched Helmholtz' chamber pipe section, and IF it is not the price of a whole new system when it is released, I am getting one for my CP-E system.