The <metadata> element is a wrapper element that contains many of the metadata elements. In topics, the <metadata> element is available in the <prolog> element. In maps, the <metadata> element is available in the <topicmeta> element.
In DITA maps, the metadata elements also are available directly in the <topicmeta> element. However, it is recommended to use the metadata elements available in the <metadata> element, as it better supports reuse between topics and maps. Collections of metadata can be shared between DITA maps and topics by using the conref or keyref mechanism.
In general, specifying metadata in a <topicmeta> element is equivalent to specifying it in the <prolog> element of a referenced topic. The value of specifying the metadata at the map level is that the topic then can be reused in other maps where different metadata might apply. Many items in the <topicmeta> element also cascade to nested <topicref> elements within the map.