Setting key names with the @keys attribute
A @keys attribute consists of one or more space-separated keys. Map
authors define keys using a <topicref> or <topicref>
specialization that contains the @keys attribute. Each key definition introduces
an identifier for a resource referenced from a map. Keys resolve to the resources given as the
@href value on the key definition <topicref> element, to
content contained within the key definition <topicref> element, or
both.
The @keys attribute is defined using the XML type NMTOKENS.
When a key is bound to a resource using @href, that resource can be a DITA
topic, DITA document, or a non-DITA resource. A key cannot resolve to sub-topic elements.
When more than one key name is specified on @keys, each key name is bound to
the same resource or resources.