The source attribute points to an external source. When used on elements describing schema components such as schemaSpec or moduleRef it identifies the source from which declarations for the components of the object being defined may be obtained.
On other elements it provides a pointer to the bibliographical source from which a quotation or citation is drawn.
In either case, the location may be provided using any form of URI, for example an absolute URI, a relative URI, or private scheme URI that is expanded to an absolute URI as documented in a prefixDef.
If more than one location is specified, the default assumption is that the required source should be obtained by combining the resources indicated.
Example
<p> <!-- ... --> As Willard McCarty (<bibl xml:id="mcc_2012">2012, p.2</bibl>) tells us, <quote source="#mcc_2012">‘Collaboration’ is a problematic and should be a contested term.</quote> <!-- ... --> </p>
<p> <!-- ... --> <quote source="#chicago_15_ed">Grammatical theories are in flux, and the more we learn, the less we seem to know.</quote> <!-- ... --> </p> <!-- ... --> <bibl xml:id="chicago_15_ed"> <title level="m">The Chicago Manual of Style</title>, <edition>15th edition</edition>. <pubPlace>Chicago</pubPlace>: <publisher>University of Chicago Press</publisher> (<date>2003</date>), <biblScope unit="page">p.147</biblScope>.