This is the tunnel instance component of the composite key identifying the specific MPLS-TE tunnel (alongside cpwVcMplsOutboundTunnelIndex) carrying this pseudowire outbound, distinguishing between multiple parallel instances of the same numbered tunnel such as primary and backup paths. It matters during TE tunnel failover events, since after a protection switchover the instance value can change to reflect that traffic is now riding the backup LSP instance rather than the primary. An engineer seeing this instance value change unexpectedly, with the VC otherwise staying up, has just found evidence of a transparent TE reroute rather than a pseudowire-level event.