FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 194718
This article discusses about security rating option which does not appear under the Security Fabric for some units.

Run a Security Fabric rating to analyze the organization's Security Fabric deployment, identify potential vulnerabilities, and highlight best practices that can be used to improve the overall security and performance of the organization's network.
The Security Fabric rating performs a variety of checks when it analyses the network.
All checks are based on the current network configuration, using real-time monitoring.
The check runs across all FortiGate units in the Security Fabric.

However sometimes the security rating option is not available under the Security Fabric.
This is because not all FortiGate models can support running the FortiGuard security rating service as a Fabric 'root' unit.
These FortiGate platforms can run the FortiGuard security rating service when added to an existing Fortinet Security Fabric managed by a supported FortiGate model.

Check in the release notes of the firmware to see the list of unit which cannot act as Fabric root unit under the FortiGuard security rating service.