A parent directory record type, used to identify the parent
directory record (superior to this directory record), or
'unknown' if no parent directory record is specified.
Usage: Supports hierarchical directory structures.
Usage: The 'Fully Qualified Distinguished Name' (FQDN) of this
directory record is the concatentation of the RDN (record key)
of this directory record with the RDNs of all superior directory
records to the 'top' (highest node).