B: The two VLAN subinterfaces can have the same VLAN ID, only if they belong to different VDOMs.
C: The two VLAN subinterfaces must have different VLAN IDs.
https://community.fortinet.com/t5/FortiGate/Technical-Note-How-to-use-emac-vlan-to-share-the-same-VL AN/ta-p/192843?externalID=FD43883 Each interface (physical or VLAN) can belong to only one VDOM.
Meaning that sub-interfaces (VLANs) from the same physical interface can have the same VLAN ID as long as they are not assign to the same VDOM.
VLAN
https://community.fortinet.com/t5/FortiGate/Technical-Tip-rules-about-VLAN-configuration-and-VDOM- interface/ta-p/197640
* VLANs can be created on any physical or aggregate (802.3ad) interfaces
- The same VLAN number cannot be configured twice on the same physical interface
- The same VLAN number can be used on different physical interfaces
- The usable VLAN ID range is from 1 to 4094
* VDOM interface assignment
- Two VDOMs cannot share the same interface or VLAN
- A VLAN sub-interface can belong to a different VDOM than the physical interface it is attached to.