These were both take from this document found online: http://www.absolute.co.jp/modules/tinyd0/content/Pixcel16052006.doc.

The 49 dot Pixel has more energy per Pixel (24 mJ/Pixel) to penetrate deeper 81 dots Pixel has less energy per Pixel (13 mJ/Pixel) for shallower treatment.

In the first treatment use the 9x 9 matrix. For the subsequent treatments use the 7 x 7 matrix. When treating the chest area use a single pass, 2 passes on the neck area and 3 passes on the face area. Treatment intervals should be on average 2 weeks apart (1-4 weeks) depends on the treatment area and patient’s healing time.

Another link: http://www.danyoplasticsurgery.com/pdf/pixel-treatment-consent.pdf

I assume that the deeper the treatment the better the treatment for scarring. As long as there are 2-3 stacks so that enough areas are being treated.