vol Volume of Related Article

Volume of the journal in which a related journal article exists.

Usage/Remarks

The @vol attribute can be used, along with the @journal-id, @issue, and @page attributes, to provide the metadata for a related journal article.
OPTIONAL on element: <related-article>
Value Meaning
Text, numbers, or special characters The journal volume number for a related journal article, for example, “3” for the third volume, “125” for the one hundred and twenty-fifth, or “XV” for the fifteenth.
Restriction @vol is an optional attribute; there is no default.
Tagged Sample

Metadata providing the volume for a Companion journal article

...
<related-article related-article-type="companion" vol="2" page="e359"
  xlink:title="synopsis" xlink:href="10.1371/journal.pbio.0020359">
 <article-title>How to Make a Mother in Five Easy Steps</article-title>
</related-article>
...