For publications that have two or more very different kinds of information, use <part> in a DITA bookmap to divide a document’s chapters into logical groupings.
Example: In a document that contains both procedures and reference information, you can define two parts, one containing the how-to procedures and the other containing the reference information.
Tip: The best practice, in general, is to reference a DITA map from the <part> element. This allows easy reuse of the collection of topics, and allows editing of the DITA map for the part in parallel to editing of the bookmap.
Click here to learn more about grouping topics in DITA.