<issn>¶
Mandatory attributes:
@pub-type='ppub'ou@pub-type='epub'
| Appears in | Frequency | 
|---|---|
| One or more times | |
| One or more times | 
ISSN is an ISO 3297:2007 numeric code which uniquely identifies a serial publication. Normally, each type of media used by a journal has its own specific ISSN.
This information can also be found in <back> within <element-citation> in the references, but no attribute is used in this case.
The allowed values for  @pub-type are:
| Value | Description | 
|---|---|
| ppub | ISSN of the print version | 
| epub | ISSN of the digital version | 
If available, both ISSN’s should be identified.
Examples:
Example of ISSN in <journal-meta>:¶
...
<journal-meta>
    ...
    <issn pub-type="epub">1712-2139</issn>
    <issn pub-type="ppub">0316-6368</issn>
    ...
</journal-meta>
...
Example of ISSN in <element-citation>:¶
...
  <element-citation publication-type="journal">
     ...
    <source>Revue de l'Université de Moncton</source>
    <volume>44</volume>
    <issue>2</issue>
    <year>2013</year>
    <issn>1712-2139</issn>
  </element-citation>
...