it's either known as a mid frame or sub frame. It holds the OLED screen and the glass digitizer. You will likely not be able to replace the digitizer/subframe/OLED individually as the OLED is generally bonded to the glass. Trying to remove it will more likely crack it. with OLED's and modern LCD's which are bonded you will need to replace the whole thing. Should not be too expensive for the 900 now though,