When creating an ADOM in Backup Mode, the ADOM is consider Read Only, where you are not able to make changes to the ADOM and managed devices from the FortiManager. Changes are made via scripts which are run on the managed device, or through the device’s Web-based Manager or CLI directly. Revisions are sent to the FortiManager when specific conditions are met:

Backup mode enables you to configure an ADOM where all the devices that are added to the ADOM will only have their configuration backed up. Configuration changes cannot be made to the devices in backup ADOM. You can push any existing revisions to managed devices. You can still monitor and review the revision history for these devices, and scripting is still allowed for pushing scripts directly to FortiGate units.