By default, when you first start up a FortiGate-7000 it is operating in Multi VDOM mode. The default Multi VDOM configuration includes the root VDOM and a management VDOM named mgmt-vdom. The management interface (mgmt) and the HA heartbeat interfaces (M1, M2) 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.
You can use the root VDOM for data traffic and you can also add more VDOMs as required, depending on your Multi VDOM license.