FortiGate models with 2 GB RAM can be a Security Fabric root
A Security Fabric topology is a tree topology consisting of a FortiGate root device and downstream devices within the mid-tier part of the tree or downstream (leaf) devices at the lowest point of the tree.
As part of improvements to reducing memory usage on FortiGate models with 2 GB RAM, FortiOS 7.4.2 and later can authorize up to five devices when serving as a Fabric root.
The affected models are the FortiGate 40F, 60E, 60F, 80E, and 90E series devices and their variants.
To confirm if your FortiGate model has 2 GB RAM, enter diagnose hardware sysinfo conserve
in the CLI and check that the total RAM value is below 2000 MB (1000 MB = 1 GB).