If you do not see the subdocument icon , click Master Document View on the Outlining toolbar.
Split a subdocument into two subdocuments
When you save the master document (master document: A "container" for a set of separate files (or subdocuments). You can use a master document to set up and manage a multipart document, such as a book with several chapters.), Microsoft Word assigns a file name to the new subdocument based on the first characters in the subdocument's heading.
If you combine subdocuments and then split them back into separate documents, Word saves the split subdocument with a new file name.