This is an internal identifier string UCS assigns to the UCS-side definition (VLAN/PVLAN and allowed vNIC type) used to generate a FabricNetwork on the VMM to correlate it with its counterpart object on the external VMM. Because the Dn and name of that network definition are meaningful only within UCS's own tree, this internal id is what UCS actually uses under the hood to match that network definition up with the corresponding object it created or found on the VMM side. A value like '42' would have no meaning to a human admin browsing the VMM's own UI, but is exactly what UCS Manager's integration code looks up internally to confirm it's talking about the same object.