I would read all the other catch-can threads. The can basically allows the condensing of the airborn oil/gas/moisture by giving the gases expansion/condensation room in the can. Some cans have filaments to aid in the condensation. That filament side would be the input (PCV side) if it has that filler inside of it. Other side (output) goes to the intake manifold. If the can is just an empty, big can, both nipples could be either input or output. Make sure to use a check valve on the intake manifold side so that boost pressure doesn't get back into the catch can when boost is building inside the manifold. Otherwise, you will be sending a big mess of fluid back to your weak sealing PCV valve, under pressure, which you don't want to do. See the other threads.
