The subtitle of the journal in which the article is published; part of the journal metadata.
Only one APA-published journal uses formal subtitles—Journal of Personality and Social Psychology. JPSP comprises three sub-journals:
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<journal-meta>
<journal-title-group>
<journal-title>Journal of Personality and Social Psychology</journal-title>
<journal-subtitle>Personality Processes and Individual Differences</journal-subtitle>
</journal-title-group>
<issn pub-type="print">0022-3514</issn>
<issn pub-type="online">1939-1315</issn>
<publisher>
<publisher-name>American Psychological Association</publisher-name>
</publisher>
</journal-meta>
...
journalmeta3.ent