value="acm_class">ACM classification</option>
              <option value="msc_class">MSC classification</option>
              <option value="report_num">Report number</option>
              <option value="paper_id">arXiv identifier</option>
              <option value="doi">DOI</option>
              <option value="orcid">ORCID</option>
              <option value="author_id">arXiv author ID</option>
              <option value="help">Help pages</option>
              <option value="full_text">Full text</