This is the name of the function/filter definition associated with this specific VM network definition reference. This matters for this row specifically when the referenced network definition relies on a named filter policy, e.g. traffic filtering rules, rather than a plain VLAN mapping; a missing or renamed fnDefName here would break that filtering behavior for just this reference. A value naming a specific filter policy would tell you this particular reference applies traffic filtering beyond plain VLAN tagging, unlike a simpler reference with this field left blank.