Aspect ratio of jpg on 1DX
If you are asking about cropping information. The file in the camera is not affected but the information is kept in the EXIF info and Canon's SW will crop the file when asked.
If using LiveView, the crop lines are superimposed while you compose a shot.
There are quite a few aspect ratios available.
See page 331 of the manual.
Not a feature I see myself using a lot.