This read-only value is the integer index that ties this RS-422 IRIG output port to its corresponding physical entity in the entPhysicalTable via entPhysicalIndex. Admins use it to cross-reference the IRIG timing output port with its physical inventory entry, useful when correlating alarms or slot/module inventory with a specific timing port. For example, a management system correlating an IRIG hardware alarm to a specific card slot would look up this index against entPhysicalIndex to find the exact physical output port.