In general, they are the same manual.
That being said, there are indeed differences between a Definity and Communications Manager.
Use the Comm Mgr manual. It will have all of the features and commands that the Definity has, it is just that the Comm Mgr does more. The screen shots will be different but it will get you over the big humps. The rest you will have to figure out on your own.
Cheers,
Brewmeister