Aspect ratios can be a confusing business. There's the Sample Aspect Ratio (SAR), also called Pixel Aspect Ratio (PAR), which specifies the dimensions of each pixel, and there is the Display Aspect Ratio, which specifies the aspect ratio of the whole picture. Not to speak of all the little details that are not quite what one'd think.
Well, basically this is simply a calculator used to determine the correct AR data for certain types of input video under certain transformations.
If you read and understood the linked page, know what you are doing, and think you found a bug in the calculations, please report it to: ar-calc a t ps - auxw d o t de
Scale transforms scale the actual video resolution to the specified size. Crop transforms crop the actual video resolution to the specified size. The target size for crop must be lower than former actual video size. Pad transforms pad the actual video resolution to the specified size. The target size for pad must be greater than the former actual video size.
The source AR is given with respect to the active picture region. This means that without transforms (like cropping to the active picture region), you won't get a DAR equal to the source AR. Do not worry. Since the vast majority of professionally mastered SD material should conform to ITU-R B.601, this is the way it's supposed to be done, using correct ITU-R B.601 PAR values.
Also note, that the correct source AR is not necessarily the AR of the movie. For example, if you have a source where a widescreen movie was encoded as letterboxed fullscreen, you'll have to choose full screen as the source AR.
About HD: It is assumed that, in the case of full HD, 4:3 is encoded pillarboxed. This way the PAR for both fullscreen and widescreen video is 1:1. For "anamorphic" HD at 1440x1080 it is assumed that 4:3 is encoded without black bars, if you select fullscreen mode. Use widescreen mode for calculations with 1440x1080 stretched to 1920x1080. In any case, for HD there do not seem to be tricky issues involving active picture regions and ITU-R B.601 PAR values, so things are pretty straightforward.