Subdocuments
Within a WordprocessingML document, it is sometimes necessary to break a large document into two or more separate WordprocessingML document files, allowing each of these files to be distributed, edited, and handled independently.
When a WordprocessingML document is comprised of other WordprocessingML documents in this way, the resulting documents are called a master document and its subdocuments.
A subdocument is a WordprocessingML document - there is no specific information in a document which classifies it as such, other than that it is incorporated into another document.
A master document is a document which incorporates one or more subdocuments (as well as optional WordprocessingML content) to create a larger document
To allow this document to be written by multiple authors, each chapter in the book is placed in a separate file (the sections highlighted in red below):
The result is three WordprocessingML documents:
A master document (containing the title of the book, the first paragraph, and references to the subdocuments for each chapter)
Two subdocuments (one for each chapter)