The edition number of a cited publication.
Any combination of:
<article>
<front>
...</front>
<body>...</body>
<back>...
<ref-list>
<title>REFERENCES</title>
...
<ref id="B6"><label>6</label>
<citation>
<collab>National Committee for Clinical Laboratory
Standards</collab>
<source>Performance standards for antimicrobial disk
susceptibility tests. Approved standard</source>
<edition>7th ed.</edition>
<year>2000</year>
<fpage>M2</fpage>
<lpage>A7</lpage>
<publisher-name>National Committee for Clinical
Laboratory Standards</publisher-name>
<publisher-loc>Wayne, Pa</publisher-loc>.</citation>
</ref>
...
</ref-list>
</back>
</article>
references.ent