This is the table index of the adjacent pseudowire segment for a multi-segment pseudowire, the programmatic counterpart to the peer PW name field used by management tools to directly join this segment's row to the next segment's full row of attributes. This is what an automated topology-tracing tool would follow, hop by hop, to reconstruct the complete chain of segments making up an end-to-end MS-PW service. As with the name field, this only carries meaningful data when the pseudowire's mode is switching, i.e. it's actually part of an MS-PW.