By "rev" I think you you are referring to the @rev attribute that is in Column view as the "Revision". This is a DITA attribute that can be set for conditional processing as a revision. Note that the attribute cascades from map to topics but there is no reverse-cascading from topics to maps. Topics are individual content containers with presumably their own revisions, etc.
The DITA Specification provides a disclaimer about using this for things like version control:
Indicates a revision level of an element that identifies when the element was added or modified. It may be used to flag outputs when it matches a run-time parameter; it cannot be used for filtering. It is not sufficient to be used for version control.If no value is specified, but the attribute is specified on an ancestor within a map or within the related-links section, the value will cascade from the closest ancestor.
Managing revision levels of a topic is certainly a tricky subject and a subject with a lot of conversation around it! Some ways I know is companies taking snapshots of all of the items and maintaining these. Certainly it is only as good as the discipline of each person to make a backup before making changes. Good luck :-)
