The turbos themselves have the ability to produce some amount of airflow at some pressure. The amount of power you can get out of some set of turbos will be differnt for each car. So to make it easy on everyone, they rate turbos for specific cars at some hp level so we know about what kind of power what turbos can make....
Example: TD05H-16Gs for 300ZX: Max power in the 600HP area... TD05H-16G for DSM: Max power in about the 350HP range...
If you want to know intrinsics about a turbo, look for cfm ratings at some psi: Turbo and Rated cfm @ 15psi: 9b - 275 14b - 430 "small" 16G - 520 20G - 650