This read-only value indicates whether a stack unit is currently physically present in the stack, true(1) if present and false(2) if not. Since a removed unit's stack and configuration data is retained rather than deleted, an admin uses this flag to distinguish between a unit that's simply unplugged versus one that's been formally removed via pdnUnitAssigned. For example, if pdnUnitPresent reads false(2) for a unit whose pdnUnitAssigned is still true(1), the admin knows that unit's configuration is preserved and waiting for the hardware to be reconnected.