16.4.4 Assigning a divider role to a section file or chapter
When filename=Part, filename logically contains all Chapters until the next Part, or the first Appendix, or the first Back item. Part may precede Chapter if the intended divider is not a file of its own; for example:
Part may precede Appendix when AllowPartAppendix=Yes (see §16.3.4 Extending <part> to include <appendix>), although this is not valid DITA 1.1.
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