XInclude is a standard for building XML documents from smaller pieces, and is an alternative to the use of external entities. The use of external entities can pose a problem because they cannot have a DOCTYPE declaration, and therefore are not valid documents on their own.
XInclude is mainly used in documentation frameworks where the documents may be composed of many files. XInclude allows you to validate the Master File as well as individual sections.
The advantages of modular documentation include reusable content units, smaller file units to be edited, better version control, and distributed authoring.
In the following images there is an example of a chapter file named chapter1.xml and a book file that uses XInclude to reference the chapter file.
Chapter file chapter1.xml:
Main book file:
The XInclude support in oXygen is turned on by default. You can turn it off by deselecting Enable XInclude processing from Options / Preferences / XML / XML Parser.