How to Add the Publication Audience to the Custom PDF Metadata
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The audience element indicates the users the publication is addressing. This can be placed inside a <topicmeta> element in a <map> as in the following example:
<map>
    ...
    <topicmeta>
        ...
        <audience type="programmer" job="programming" experiencelevel="expert"/>

To collect the @type attribute, add the following in your customization css:

map > *[class ~= "map/topicmeta"] > *[class ~= "topic/audience"] {
    -oxy-pdf-meta-custom: "Audience" attr(type);
}
Note: It is best to use the class selector (such as *[class ~= "map/topicmeta"]) instead of topicmeta to cover cases where the elements are specialized (for instance, in a bookmap the bookmeta is a topicmeta, so your selector will also function for bookmaps, not only simple maps.
Note: The selector begins with map > to choose the <topicmeta> that is a direct child of the map, not other <topicmeta> elements from other <topicref> elements.

The metadata is displayed in the Custom tab of the Document Properties dialog box from Acrobat Reader: