'clean' and 'dirty' are non-technical names for the two sides of the PCV system. Clean (air)= System Input, Dirty (air)= System output
The clean side connects at the throttle body and valve cover for a N/A LSx. This side is designed to flow fresh filtered air from the air intake (atmosphere) into the engine crankcase.
The dirty side connects from the valley cover or opposing valve cover to the intake manifold plenum chamber (engine vacuum).
Blowby can flow out the clean side when dirty side flow is insufficient to overcome blowby production...
+1 on getting a better catch can.