mdb / rex

MDB (Movie Data Base) adds to your epg for movies or series a complete set of additional element, after grabbing epg the post process search movie database, imdb is the most complete and based on the search add additional actors, comments, star rating etc. etc. elements that you define in a mdb.config.xml The search must have as input good elements example, if i search a serie, title, director but my epg has no episode number the search is = 0, the same for movies: you need in your epg at least title, director and productiondate.

Rex is another post process, that modify the epg output as you like. Example on satellite receiver there is no standard view for elements (You have time, title and description) with rex you can modify description and add productiondate, actors, etc etc. as you prefer and define in rex.config.xml.

Both process will leave your original xml and create a new one with the modification.
Btw it's all in documentation, samples are in the documented configs or you can search the forum.

yes(usually).

the original epg.xml file webgrab creates during grabbing remains unchanged.
the modified version created during rex postprocessor is a different file.
this is why it specifically says in the docs that these 2 filenames must be different.