DM is a guide and a helping hand to familiarize you of the dive sites your about to dive. Also can be wealth of info on local history. Other than that you can request what more you want, for instance help with gear or gear issues, boat exit and entry, to what ever you request in aide of your diving.
They are not instructing you as a paid class, instructors that are DM's have no responsibility of you. If you are paying an Inst/DM for a class then they have responsibility of you for that particular dive.
For instance if you do a refresher course and do first dive, after debriefing of that dive, and you do a second dive he is not responsible for you, as he is guiding you on the second dive.
Now I believe a DM can perform a refresher course, so it is the same, as with any thing else a DM can now teach.