This dialog allows you to select between "Standard" and "CCIR601" proportions.
The Standard setting results delivers square pixels on a standard dedicated monitor that has a normal aspect ratio (ratio of height to width).
CCIR601 results in a full-screen line length of 720 pixels in both NTSC and PAL/SECAM modes. In the NTSC format, the video when viewed on a standard monitor will have narrower than square pixels. In PAL/SECAM format, it will have wider than square pixels. When viewed in a video window on your main screen, the pixels will be square, but the aspect ratio of the frame as a whole will be different from the standard wider for NTSC, narrower for PAL/SECAM.