its a through hole to the inside of the crankcase, oil mist/pcv vapor will exit it if no blocking bolt is installed.

m10x1.5 thread, less than 15mm uhl/thread length. that hole is used on the older g/h body engine mounts that make it a pita to change the belts.

each cylinder head also has a through hole near the injectors that is only used on certain models/mounts/accessories that provide the same leak path from the crankcase to the outside world.