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From the menu File → Open FITS file.
This let you load a FITS file in the preview window instead of taking the image with the camera.
This is useful if you want to review a previous shot from the capture sequence, or to play with some other image.
Note CCDciel is not a multi-purpose FITS file viewer. It is intended to display images from the camera but it could fail displaying FITS files from other programs.
There is also chance CCDciel cannot plate solve a file taken with another software if WCStools cannot find the approximate coordinates and scale of the image.