1) If a device is hidden ( https://docs.fortinet.com/document/fortimanager/7.4.2/administration-guide/115931/hiding-unauthorized-devices), then it's not used in a device license count and FMG doesn't provide FGD service to that device,
2) If the device isn't hidden, but unregistered and unreg_dev_opt add_allow_service is disabled, then it's counted in a device license count and FMG doesn't provide FGD service to that device,
3) If the device isn't hidden, but unregistered and unreg_dev_opt add_allow_service is enabled, then it's counted in a device license count and FMG provides FGD service to that device,
4) This was changed as of FMG 6.2.0GA
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So to allow FGD service for unregistered devices it's a must to have the:
- unreg_dev_opt add_allow_service is enabled
- sufficient device/domain license
