In formula car use, the engine is dry sumped, and should be at a slight neg pre. in the block, so a plate is made that just blocks off the hole. Usually a vent line will be run from the cam cover to the oil tank, but Dave Bean did make a block breather cover, that had a tube welded in, to use as a vent line to the oil tank, installed over the hole in your block.
THis is for all of you who are still laughling at me - the hole, of course, is in the head, not the block - and it is a cam cover, not a valve cover. I'm getting old.
Not laughing. There is the hole in the left rear side of the head that vents to the air box on road cars. There is also a hole directly beneath the one on the side that drains oil from the head back to the block with a special flanged rubber hose which turns hard and leaks.
Depending on the engine builder, some use the drain hole in the head to the block, others plug it.
__________________ "Driving a Lotus is a triumph of bravery over intelligence." Stirling Moss