A directory record index, used to uniquely identify this
directory record (and all subordinate directory attributes),
when combined with 'xcmCommsDirRecordType' (above).
Usage: This directory record index SHALL be unique for
each record of a given type (value of 'xcmCommsDirRecordType'),
but NEED NOT be unique for directory records of different types.
This is to permit directory records of different types to start
their numbering (value of 'xcmCommsDirRecordIndex') over from
one ('1') and such usage is RECOMMENDED.