A master document is a Word file that contains links to a set of other, separate Word files, called subdocuments. The content of the subdocuments is not inserted into the master document. The master document only contains links to the subdocuments. This allows you to edit the subdocuments separately. Any changes made to subdocuments are incorporated into the master document automatically. If there are multiple people working on a single document, a master document allows you to send different parts of the document to different people to work on.

Read More: Create a Master Document in Word 2010 from Multiple Documents