When set, this always wins over "Host Selection Mode" below, which is then ignored. Leave blank to use vCenter's own default placement (unchanged legacy behaviour), or to let "Host Selection Mode" choose automatically.
Works on any vSphere edition/license, and with any permission setup - including a service account that can only write to a single host. This is usually the right choice for small/test labs, or whenever you want full manual control over where clones land.
Example: esx-rack3-host07.example.com