Which downstream FortiGate VDOM is used to join the Security Fabric when split-task VDOM is enabled

Which downstream FortiGate VDOM is used to join the Security Fabric when split-task VDOM is enabled on all FortiGate devices?

A. FG-traffic VDOM

B. Root VDOM

C. Customer VDOM
D. Global VDOM

Reference: https://docs.fortinet.com/document/fortigate/6.2.0/new-features/287377/split-task-vdom-support