For a vcOnly pseudowire (no outer tunnel), this holds the ifIndex of the physical or logical interface on which this router expects to receive the VC's labeled traffic, and it's zero whenever an outer tunnel exists instead. It's the direct counterpart to cpwVcMplsOutboundIfIndex and is the first place to check when inbound traffic for a vcOnly VC isn't arriving, since it tells you exactly which receive interface's link state and error counters are relevant. If this ifIndex resolves to an interface showing input errors or a down link, that directly explains inbound failures on this specific pseudowire without needing to suspect the label or the PSN core.