Like atiVlanIfIndex, this is the ifIndex value assigned to a new VLAN encapsulation interface rather than a plain VLAN interface, and it too must be obtained in advance from atiNextAvailableVirtIfIndex before being written here to avoid colliding with an existing entry. It matters specifically for interfaces that need tag-based encapsulation layered on top of a physical port, which is a separate table from plain VLAN interfaces in this MIB. An NMS setting up a trunk encapsulation would fetch the next free virtual index and write it here as the first step of creating that encapsulation row.