VideoCrop

How many rows and columns at each edge of a stream's decoded pictures are not part of the image, as the container says: a Matroska track's PixelCrop elements, or an MP4 track's clean aperture. Encoders use it to show 1080 lines of a 1088-line coded picture, cameras to hide sensor margins, and remuxers to cut black bars without encoding again.

FFmpeg reads it and does not apply it, so a renderer shows the picture without top rows at the top, bottom at the bottom, left columns at the left and right at the right, and takes the display aspect from what is left. The crop that the bitstream itself carries, such as the cropping of an H.264 sequence parameter set, is a different thing: the decoder applies that one before a frame arrives, and it is not here.