This is the Distinguished Name identifying, within UCS Manager's management tree, the equipment-definition record that describes this class of power supply -- its physical and electrical specification rather than the live installed unit. Because equipment definitions are shared reference data (one definition can apply to every PSU of that exact model across a whole UCS domain), this dn lets an admin or script pull up the spec-sheet-like definition object independently of which chassis or bay a matching physical PSU happens to sit in. A typical value looks like sys/chassis-1/psu-2/def, pointing at the definition record associated with the PSU installed in bay 2 of chassis 1.