A directory record key attribute string value, used to specify
a primary key attribute for sorting this directory record, or
zero ('0') if no primary key attribute is specified.
Usage: String-valued directory attributes SHALL be
specified as UTF-8 encoded Unicode (ISO 10646) strings.
Usage: The 'Relative Distinguished Name' (RDN) of this
directory record is the tuple of the directory record type, the
key attribute type, and the key attribute value.