The <topicref> element, and any specialization of
<topicref> that allows the @keys attribute, can be used
to define keys.
In the following example, a <topicref> element is used to define a key;
the <topicref> element also contributes to the navigation structure.
<map>
<!--... -->
<topicref keys="apple-definition" href="apple-gloss-en-US.dita" />
<!--... -->
</map>The presence of the @keys attribute does not affect how the
<topicref> element is processed.
In the following example, a <keydef> element is used to define a
key.
<map>
<!--... -->
<keydef keys="apple-definition" href="apple-gloss-en-US.dita"/>
<!--... -->
</map>Because the <keydef> element sets the default value of the
@processing-role attribute to "resource-only", the key definition does not
contribute to the map navigation structure; it only serves as a key definition for the key name "apple-definition".