This item is an identifier for the convention followed in
              constructing the contents of _array_data.header_contents
              The permitted values are of an image creator identifier
              followed by an underscore and a version string.  To avoid
              confusion about conventions, all creator identifiers
              should be registered with the IUCr and the conventions
              for all identifiers and versions should be posted on
              the MEDSBIO.org web site.