After I made edits in Track Changes, I saved the subdocument and then opened
the master document. The subdocument was locked within the master document.
I expanded the subdocuments, located the the one I needed, and wanted to
unlock it. The icon on the Outlining toolbar shows that I can do this (it's
not gray). But when i click on the Lock/Unlock icon to unlock the
subdocument, this message occurs:

"The operation cannot be complete because the subdocument has a different
type of protection from the master document. Make the protection the same
for the master document and subdocument."

What does this mean and how can I change the protection?
Thank you!