[HKSevo]

pretty much sums it all up from the common name of the parts


turbo back is basically all of the exhaust to and from the turbo execpt the headers(downpipe, cat back, cat/test pipe)

cat back is basically the axle back...(connected to the cat)



the turbo back would most likely be in separate pieces, including a cat back
Both should be the same diameter but it all it matters what brand or size u buy ofcourse