Another (simple) PCV Question

[lanky 44]

So I understand how a pcv system evacuates crankcase vapors, but I still have a question:

1) What is the difference (in terms of function) between an oil filler/breather)cap that:

       A) IS vented (holes in bottom of it) -- connected to a hose to clean-side of air cleaner

       B) IS NOT vented (no holes underneath)--- connected to a hose to clean-side of air cleaner

Since this is the side of the pcv system that fresh air is drawn through, wouldn't you want all of that air to be filtered?

Or are those holes there to escape sudden rises in crankcase pressure (if that even happens)? Seems like the holes would increase chances of a leak/mist.

To all you out there, yes I am referring to the side that DOESN'T have the pcv valve. I understand the pcv valve gets connected to manifold vacuum ideally at the carb base.