An ISSN designated by an ISSN Centre to group together the different media versions of a work. Since a single ISSN-L is assigned no matter how many media formats of a work there are, the ISSN-L can be used to link among the different media versions of the work or continuing resource, for example, both the printed and online versions of the work.
<!ELEMENT issn-l (#PCDATA %issn-l-elements;)* >
(#PCDATA)*
Text, numbers, or special characters, zero or more
<element-citation>, <journal-meta>, <mixed-citation>, <product>, <related-article>, <related-object>
...
<journal-meta>
<journal-id journal-id-type="pmc">pnas</journal-id>
<journal-id journal-id-type="pubmed">Proc Natl Acad Sci U S A</journal-id>
<journal-id journal-id-type="publisher">PNAS</journal-id>
<issn pub-type="ppub">0027-8424</issn>
<issn pub-type="epub">1091-6490</issn>
<issn-l>0027-8424</issn-l>
<publisher>
<publisher-name>The National Academy of Sciences</publisher-name>
</publisher>
</journal-meta>
...
JATS-common1.ent