Im, my understanding is that some batting come with a "scrim" a fine layer/webbing that is meant to keep the batting fibres more stable and together. I think in theory it enables a wider quilting space but it's also useful to prevent the needle punching wisps of wadding through your fabric. If you place your batting "scrim side" down, ie against your backing then supposedly when the needle punches down from the top fabric and pierces through the batting and then the backing, the scrim helps prevent the needle pushing wisps through as it completes its stitch. If the scrim is "up" against the underside of the top fabric then the needle is passing through the batting AFTER it has gone through the scrim so there is nothing to stop any wisps being carried through from the batting out the backing stitch hole. Now, some battings don't "wisp" as much as others, and obviously if you have a dark/charcoal coloured batting with your dark backing fabric, wisps aren't going to be so noticeable.
There....clear as mud,,,simples....well, that's my understanding of the process, I am happy to stand corrected if I am wrong.
Hi Efemera, my batting is a 60/40 wool poly...white...ugh....I knew I should've bought some dark batting but thought I could get away with the white as I have heaps of it if I laid the scrim correctly...duh.......