FortiManager tries to obtain updates from the servers configured in the Use Override Server Address for FortiGate/FortiMail section. Depending on the Server Override Mode configuration, you can restrict FortiManager to receiving updates from the configured override servers list, or allowing fallback to other public FDS servers, if FortiManager is not able to communicate and receive updates from the configured server list. In the example shown on this slide, two override server addresses are configured. When Server Override Mode is set to Strict, FortiManager gets updates only from these two servers. There is no fallback to other public servers, if these two configured servers are not available. If you set Server Override Mode to Loose, FortiManager first tries to get updates from the configured server list; if they are unavailable, FortiManager falls back to other public FDS servers to get updates. Reference [Page 357] - https://ebin.pub/fortinet-fortimanager-study-guide-for-fortimanager-72.html