This window allows you to copy an astrometric calibration from a file to the displayed image. It works just like the usual file selection window except for the additional buttons displayed at the bottom.
When you have selected the file you want to copy the calibration from try the "Test" button and see the RA and Dec coordinates that are now available (you can also try to display catalogue information -- just to see how good the copied calibration is). When you're happy press the "Accept" button to make the change permanent (but remember to save the image to keep the new calibration permanently). You can now use the "Tweak an existing calibration..." toolbox to add a linear transformation (i.e. an offset, scale change, rotation etc.), if necessary (if say the copied calibration is from a slightly dithered image).