This button holder located in the browsing section, on the right side of the screen on CDJ2000NXS2, XDJ XZ, XDJ RX2 and more high end models from Pioneer DJ. The button holder is keeping the four buttons located arround the browse encoder in place.
Use left/right arrows to navigate the slideshow or swipe left/right if using a mobile device
Choosing a selection results in a full page refresh.
Press the space key then arrow keys to make a selection.