This read-only INTEGER reports which other stack unit, by number, is physically connected to this unit's left-side Hyper GLink stacking port, or 0 if nothing is connected there. An admin diagnosing a broken stack ring topology uses this and its counterpart, rlPhdStackConnect2, to verify the physical cabling matches the expected ring, since a 0 where a neighbor should be indicates a disconnected or failed stacking cable. For example, if unit 2 is expected to connect to unit 1 on its left side but this field reports 0, the admin knows to check the physical stacking cable between those two units.