As the picture aspect ratio example indicates, videos are series of pictures that do not necessarily have the same aspect ratio as the final result to be displayed to the user.
So how do you get from those stretched pictures to the normally displayed output? You need a stretching factor! This stretching factor is applied to the picture in order to bring it to the correct aspect ratio for display. This is the sample aspect ratio or SAR.
The stretching factor is often expressed as a ratio of two integers. You calculate it as SAR = PAR / DAR.
From the illustrated parameters, we see that SAR = 9:16 / 16:9 = (9/16)/(16/9) = 81/256 = 3.1604938271604938271604938271605