All clips and images have a pixel aspect ratio, this is how much a 'real' pixel must be stretched by in X to be square. For example PAL D1 images have a pixel aspect ratio of 1.06666.
is used to control how a plugin deals with pixel aspect ratios. This is both a host and plugin property. For a host it can be set to...
For a plugin it can be set to...
If a plugin does not accept clips of differing PARs, then the host must resample all images fed to that effect to agree with the output's PAR.
If a plugin does accept clips of differing PARs, it will need to specify the output clip's PAR in the