Classification domain elements
The classification domain elements are used to identify
         the subject matter of content that is referenced in a map. These subjects
         must be subjects defined in a subject scheme map. In particular, this
         allows an author to classify content in new subject categories that
         are not bound to existing metadata attributes.
      
      - <subjectref>The <subjectref> element identifies a subject to classify content. The <subjectref> can identify the subject with a @keyref attribute (if the scheme has a <subjectdef> with a @keys attribute that assigns a key to the subject) or an @href attribute (if the scheme is not available and a topic exists that defines the subject).
 - <topicapply>The <topicapply> element identifies subjects that qualify the content for filtering or flagging but not retrieval. The <topicapply> element can identify a single subject. Additional subjects can be specified by nested <subjectref> elements.
 - <topicsubject>The <topicsubject> element identifies the subjects covered by a topic or map.
 - <topicSubjectTable>The <topicSubjectTable> element is a specialized relationship table which allows a map to use relationship tables to associate topics with subjects. Tools (such as search tools) might use these classifications to retrieve content that is relative to a specific subject or combination of subjects.
 - <topicSubjectHeader>The <topicSubjectHeader> element specifies constraints on the subjects used in classifications.
 - <topicSubjectRow>The <topicSubjectRow> is a grouping element that contains one row of a subject table. It contains topic references in the first column, and relates those references to the subjects in each following column.
 - <topicCell>The <topicCell> element contains topics that will be associated with subjects in each following column of the current row in the <topicSubjectTable>.
 - <subjectCell>The <subjectCell> element contains subjects that are associated with topics in the first column of the current row in the <topicSubjectTable>. The subjects themselves have no defined relationship across columns, other than the fact that they apply to the same content.
 
Parent topic: Classification elements