(Optional, a whole number from 0 to 100) A host with less than this share of its own CPU capacity free is kept off the list of candidates for automatic host selection.

This is relative to each host's size, so it suits farms of mixed hosts: 20 means "do not put a new VM on a host that is already more than 80% busy". Zero (the default) switches this limit off. See the minimal free CPU in MHz for the absolute variant and for how the limits combine.