"Back room" -- I like that! I suppose the general idea is that there's this place somewhere out out "back" where the client can't see it where you administer things.
I usually call it the admin area but I have no idea why I'm bothered by the use of "back office" (English words) in a French language environment, when the term is not too common in English in this context. (Or that's the hypothesis I'm trying to test/challenge).