The identification information (typically the standard number, organization name, and title of the standard) for a cited standard, where “standard” is defined as a document produced by a recognized standards body such ISO, IEEE, OASIS, ANSI, etc.
<!ELEMENT std (#PCDATA %std-elements;)* >
Any combination of:
<citation> Citation; <product> Product Information; <related-article> Related Article Information
...
<ref>
<citation>
<std>International standard ISO 10993-10:2002(E): Biological
evaluation of medical devices-Part 10: Tests for irritation and
delayed-type hypersensitivity</std>. <edition>Second</edition>.
<year>2002</year>-<month>09</month>-<day>01</day>
</citation>
</ref>
...
references.ent