This read-only InterfaceIndexOrZero identifies the outgoing interface index associated with a policy-based routing rule's next-hop; the underlying source description text for this object appears to duplicate a neighboring field's wording, so this reflects the object's own name and integer-interface-index type rather than additional detail. In general, it lets the admin see which interface PBR-selected traffic is actually sent out through. For example, this field pointing to a specific WAN interface's ifIndex would show which physical or logical port PBR-selected traffic actually egresses through.