By default, when you first start up a FortiGate-7121F 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 interfaces and the HA heartbeat interfaces are in mgmt-vdom and all 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 you can create LAGs that include 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.