This read-create Integer32 is the index that identifies a specific entry in the dpPortBasedVlanTable, i.e., a particular port-based VLAN. Because it's the table's sole index, an admin uses it to create, reference, or delete a given port-based VLAN's row, matching it up with its egress-port list and row status. For instance, creating a new port-based VLAN entry with dpPortBasedVlanIndex set to 20 establishes VLAN 20 as a distinct row the admin can then populate with egress ports.