Within a bibliographic reference (<citation>), the <source> is the title of a journal, book, conference proceedings, etc. that contains (is the source of) the material that is being cited.
Any combination of:
Title of a book
<back>...
<ref-list>
<ref id="B1">
<label>1</label>
<citation><collab>Research Unit of the Royal
College of Physicians and British Geriatric Society</collab>
<source>Geriatric day hospitals: their role
and guidelines for good practice.</source>
<year>1994</year>
<publisher-name>RCP</publisher-name>
<publisher-loc>London</publisher-loc>.
</citation>
</ref>
...</ref-list>
...</back>
Title of a journal
<back>
...
<ref-list>...
<ref id="B8"><label>8</label>
<citation>
<name><surname>Weissert</surname>
<given-names>W</given-names></name>
<name><surname>Wan</surname>
<given-names>T</given-names></name>
<name><surname>Livieratos</surname>
<given-names>B</given-names></name>
<name><surname>Katz</surname>
<given-names>S</given-names></name>
<article-title>Effects and costs of day-care
services for the chronically ill: a randomized
experiment.</article-title>
<source>Medical Care</source>
<volume>18</volume>
<year>1980</year>
<fpage>567</fpage>
<lpage>584</lpage>.
<pub-id pub-id-type="pmid">6772889</pub-id>
</citation>
</ref>
...</ref-list>
</back>
references.ent