FortiGate Security 7.2 Study Guide (p.436): "When you configure FortiGate devices in multi-vdom mode and add them to the Security Fabric, each VDOM with its assigned ports is displayed when one or more devices are detected. Only the ports with discovered and connected devices appear in the Security Fabric view and, because of this, you must enable Device Detection on ports you want to have displayed in the Security Fabric. VDOMs without ports with connected devices are not displayed. All VDOMs configured must be part of a single Security Fabric."