Glasspack mufflers do not have extra sound-absorbing material. So, in order to absorb sound, they use the glass layer and packing. But if you are making your order on partcatalog.com, you can always inquire about the recommended tube diameter for your exhaust system by providing your make and model. This can lead to a significant decrease in torque and horsepower as well as rpm.
Cat-back exhaust systems will include any pipes that run from the catalytic converter to the muffler, the muffler itself, any pipes that may run from the muffler to the exhaust tips, and the exhaust tips themselves. Some cat-back exhaust systems have a muffler egr delete 2005 duramax, which means no muffler is included. This is typically found on vehicles that function primarily for off-road use