This links this pseudowire's outbound entry to the specific cross-connect (XC) entry in the separate MPLS-LSR-MIB that represents the outer LDP or manually-configured tunnel carrying the VC label, reading zero if there's no such outer tunnel. It's the pointer an engineer follows to jump from pseudowire-level troubleshooting into the underlying LSR cross-connect table when the outer tunnel itself, rather than the VC, is suspected to be the problem. A nonzero value here that no longer resolves to a valid entry in the MPLS-LSR-MIB's cross-connect table indicates the outer tunnel was torn down out from under this pseudowire.