Ajax is the expert on anhydrite screed on the forum ,but in laymans terms anhydrite screeds are gypsum based rather than cement based ,they are normally flowable and pumped in and give a very level flat finish when done right ,these types of screeds need different type of preparation to conventional sand and cement based floors and the surface latience is normally ground off prior to tiling ,anhydrite screeds are not normally recommended for wet areas due to their chemical compostion also they must be primed prior to the installation of tiles as most cement based adhesive react badly to this kind of surface, anhydrite screeds are extremely popular in europe and are slowly becoming popular here ,if you are unsure about a substrate you are tiling on whether its anhydrite or not i would recommend decoupling it with a matting system, hope this helps, i am sure ajax will be along shortly to clear up any confusion