Quartz filters in ColorSync Utility: what is "Image Sampling" and Quality? I have created a couple of quartz filters to reduce the size of PDFs. For this I use Image Compression - which is pretty straight forward (just use JPEG and the a suitable quality level) - but there is also a filter called Image Sampling that also offers a quality level (low, medium, high).

I have tried it but I don't notice any significant difference. On what quality of the PDF is this option supposed to apply?

The other alternatives (size, resolution, scaling) in the Image Sampling filter are straightforward but I don't understand why there is a quality option in both Image Sampling and Image Compression.
As Apple's documentation on Quartz Filters is almost non-existent, it's hard to be certain. However, given the nature of the component, the "Quality" of the re-sampling is likely to relate to what algorithm is used to re-resolve the image.

If you've ever re-sampled an image in Photoshop, different algorithms are used to perform the mathematical transformation, such as Bicubic, Bilinear, and Nearest Neighbour. The differences may be subtle, or not, depending on the nature of the image and the change in size.

Quality in Image Compression relates to how lossy the compression will be, i.e. how much data is thrown away to reduce file size.

(If the images in the PDF are already low-res and also JPEG, then you may not see much further reduction in file size or image quality.)