According to ETNews, Samsung aims to expand the adoption of its Y-OCTA touch technology which will be used in both versions of its Galaxy S9 (5.77" and 7.22"). In the Galaxy S8, only the 5.77" version uses Y-OCTA, the larger variant uses Samsung's film-type touch.
Y-OCTA (which apparently stands for Youm On-Cell Touch AMOLED) describes Samsung's On-Cell flexible (hence Youm) AMOLED touch technology. The touch sensor in Y-OCTA displays is deposited directly on the encapsulation (TFE) layer which is better than the add-on (or film-type) touch used in older flexible AMOLEDs generations. The optical features are better as the touch layer is below a polarizer and enables the use of a non-ITO grid, there's no need for a support film (see image above) and the number of layers is lower. Samsung estimates that Y-OCTA also cuts production costs by around 30%.