By default, when you first start up a FortiGate-6000F it is operating in Multi VDOM mode. The default Multi VDOM configuration includes the root VDOM and a management VDOM named mgmt-vdom. The mgmt1, mgmt2, mgmt3, ha1, and ha2 interfaces are in mgmt-vdom and all of the data interfaces are in the root VDOM.
You cannot delete or rename mgmt-vdom. You also cannot remove interfaces from it or add interfaces to it. You can however, configure other settings such as routing required for management communication, interface IP addresses, and so on. You can also add VLANs to the interfaces in mgmt-vdom and create a LAG that includes the mgmt1 and mgmt2 interfaces.
You can use the root VDOM for data traffic and you can also add more VDOMs as required, depending on your Multi VDOM license.