there's no wiki, but i would guess bare_back refers to completely exposed/unclothed back of a character (with no dangling articles of clothing) while back refers to the posterior part of the torso (not necessarily exposed) like post #5113 and post #10278.
however, it seems like both tags need massive cleaning (if even it's practical) to delineate the two. i'm not so sure now if that's the original intention of the tag usage.
Updated by ghostrigger