This read-only integer identifies, via the standard ENTITY-MIB entPhysicalIndex, the physical port on the parent switch that connects a bound port extender back to the chassis, reading zero if the port isn't bound or isn't up. Admins use it to trace the physical cabling path from a PEX port back to its uplink interface on the switch. For instance, cross-referencing this value against the ENTITY-MIB entPhysicalDescr table tells an admin exactly which uplink port a misbehaving PEX is physically connected through.