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Contributed by: Radu Coravu

You can merge multiple existing DITA Maps in various new publications.

The only existing danger for this would be if you define keys with the same name but different values in both publications. Fortunately DITA 1.3 comes to the rescue with the new keyscopes support which allows keys with the same name to be resolved to various values on each scope:
<!DOCTYPE map PUBLIC "-//OASIS//DTD DITA Map//EN" "map.dtd">
<map>
    <title>Vegetables Soup</title>
    <topicref href="carrots/carrots.ditamap" format="ditamap" keyscope="ks1"/>
    <topicref href="potatoes/potatoes.ditamap" format="ditamap" keyscope="ks2"/>
</map>

Even if you have a single root map you can keep related sections or chapters in different DITA Maps. Besides adding more logical structure to your content you never know when you'll reuse those sub-maps in different publications.