You can break up a large DITA map into more manageable pieces by creating sub-maps. A sub-map is simply a DITA map that is included by another DITA map. There is no separate markup for a sub-map.
For example, if you are creating a book, you might use one sub-map for each chapter of the book. If you are reusing a set of topics in multiple publications, you might collect them into a map and reuse the map as a sub-map in multiple other maps, rather than referencing the topics individually from the new maps.
You add a sub-map to a map the same way that you would add a new topic or insert an exiting topic into a map, except you choose a map rather than a topic to create or add. When adding a sub-map, to a map, make sure that you use a mapref element or a topicref element with the format attribute set to ditamap. In most cases, Oxygen XML Editor takes care of this for you.
You can move sub-maps around in a map just as you would move a topic.