(Optional, a whole number) A host with less than this much free RAM, in MB, is kept off the list of candidates for automatic host selection.

Zero (the default) switches this limit off. It is an absolute amount; see the percentage limit for mixed farms, and the minimal free CPU in MHz for how the limits combine. Unlike Require enough free RAM, which compares with the size of the VM being created, this is a fixed amount that admins decide to always keep free on a host.