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.
|Valley by the light of a blue moon by cjf2|
from Down in the Valley
|Lake Erie Stone Pier by yobbyt|
from Dock or Pier