From Word HELP:
[[Lock or unlock a subdocument
On the View menu, click Outline.
On the Outlining toolbar (toolbar: A bar with buttons and options that you
use to carry out commands. To display a toolbar, use the Customize dialog
box (point to Toolbars on the View menu and click Customize). To see more
buttons, click Toolbar Options at the end of the toolbar.), click Expand
Subdocuments .
Click anywhere in the subdocument you want to lock or unlock.
On the Outlining toolbar, click Lock Document .
Notes
Microsoft Word also locks subdocuments in the following cases:
The subdocuments are collapsed (collapse a master document: Collapse a
master document in outline view to hide the contents of subdocuments and
display them as hyperlinks.).
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.) is shared by a
workgroup, and someone else is currently working on the subdocument.
Someone who previously worked on the subdocument set file sharing to the
read-only (read-only: A setting that allows a file to be read or copied, but
not changed or saved. If you change a read-only file, you can save your
changes only if you give the document a new name.) option.
The subdocument is stored on a read-only file share.]]
