Combining Rendered Scenes and Real Life Footage: Which one should be (un)distorted?

When combining rendered scenes with real life footage, the question arises whether the footage should be undistorted or the scene should be distorted using the calculated or manually set real life camera properties from the "Movie Clip Editor".  The "Blender 2.93 Manual" here gives an explanation but I'm still looking for a more detailed explanation concerning the "extra pixel information" and "extra unavailable pixels" which are mentioned in the manual.