I would say it's just the name. Here is the info from their website.
Available as a full system, or as a header system for riders who are already using Rinehart Racing

slip-on mufflers. The Slimline Duals

header system is built with the rider in mind, with a perfect combination of performance and style to maintain the classic look. Offered in both chrome and black, with chrome and black end caps, you can choose which style fits you best. Want to customize your exhaust even further? Check out our MOTO Series

end caps as well.
Our Slimline Duals

are built to maximize performance, featuring anti-reversion louvers that stop the negative pulse from traveling back into the port, which would otherwise rob your bike of horse power. The Slimline Duals

also offer 12mm and 18mm O2 ports, placed perfectly for dyno tuning and proper measuring of exhaust gas concentration, maximizing performance efficiency.
Performance isn’t the only upside either. We also designed them with rider comfort in mind. The Slimline Duals

are designed in such a way so they actually point the head pipes away from the rider’s legs, keeping the heat away and maximizing comfort, especially over long rides.
2016 CVO Street Glide
Carbon Crystal w/ Phantom Flames
Matched Chopped Tour Pack
Reinhart Slimlines
Fuel Pak FP3
Screamin Eagle Breather
Yaffy 12" Bars and License Plate Frame