Cascading of metadata elements from map to map
Elements that are contained within <topicmeta> elements follow
        the same rules for cascading from map to map as the rules that apply within a single DITA
        map.
For a complete list of which elements cascade within a map, see the column "Does it cascade to child <topicref> elements?" in the topic Reconciling topic and map metadata elements.
Note (non-normative): 
    It is possible that a specialization might define metadata that
      is intended to replace rather than add to metadata in the referenced
      map, but DITA, by default, does not have a
        mechanism to specify this behavior.