Whichever rooms you have the DVR's in, can pause live tv of course. Whatever rooms you have a standard HD box on MRV, can only playback a recording off the HDDVR. Say, if you wanted 4 tuners for recording, and wanted ability to watch recorded shows in all 4 rooms, you'd need 2 HDDVR's and 2 HD boxes. The non DVR's can playback content off one of the DVR's in the MRV chain, but they cannot do anything with live tv, or record. They are merely just a window to recording on how many ever HDDVR's you have.
I needed 4 tuners, and only have two tv rooms, so I logically got two DVR's (2 tuners each). If I only needed 2 tuners, I woulda gotten a DVR for the Liv Room and a HD box for the Bedroom.
With MRV connected boxes, you can watch and delete off any box, from another, but only the DVR's can record. They also have not combined series links yet, so I have some timers/recordings set in the Bedroom, and some in the Liv Room.