This window is designed to allow you to select the built-in coordinate system shown in the alpha and delta readouts in the main window (the secondary coordinates).
The "System:" drop down menu shows all the systems that are available to you. Just select one to make this the current secondary coordinate system. To make this change permanent just close the window using the "Accept" button. The "Reset" button switches back to the original system and "Cancel" closes the window after a reset.
Usually FITS images will have only two built-in coordinate systems, the base (or grid) one shown in the X and Y readouts and a celestial one, shown in the alpha and delta readouts. However, future standards may provide additional systems that describe things like the coordinates of each chip in a mosaic camera and the coordinates of the camera as a whole, as well as some celestial system.
NDF images have several coordinate systems associated with them: