Let me add to that
In what I call a "semi-closed" system, you would have
1- A PCV connected to a full time (manifold) vacuum source that would evacuate during idle and part throttle
2- A second tube, usually a filtered breather tube that is an air source at light throttle, and when vacuum is at zero, could relieve pressure into the air cleaner
For normal street/hot rod systems you have #1, but number 2 is just an open breather.
Keep in mind, the "semi-closed" system really is a benefit for emissions as any blowby, in any case, gets reintroduced into the system.