Audio discs, acetates, studio discs, emidiscs are all basically the same. They are wax cover steel discs, used to cut one off or small runs of song(s). This was done for several reasons, as a quality check of a tune before going to pressing, as an advance promotional tool, to be given to record company executives so they could listen a give views or for unsigned artist(s) to send record companies.
Different people use different terms but they all have the same meaning, except Emidisc. Although it is the same type of disc, it is a brand of disc, that was used in this counrty both by professionals and by bootleggers. It has become the common term on the soul scene for an illegal copy on one of these discs of a tune. So is given little value.
Otherwise these studio discs could have great value as somethimes they can be of a different version of the released track, or a completely unreleased tune.