The attribute revisionflag is usefull to identify the changes between two revisions of a document. This information is managed by dblatex, that adds revision bars in the margin of the paragraphs changed, such like in this paragraph.

Adding the revision flags can be manual, but its is tedious and error prone. The diffmk tool by Norman Walsh can do the work for you. See http://www.sagehill.net/docbookxsl/Changebars.html for more details about how to use it.