The #7 is a polish that removes oxidation, bonded contaminents, etc. By using it first it "deep cleans" the trim so that it can be dressed with Meguiar's plastic polish or Vinylex. (or whatever your preference is) I've found that after a good application of a polish like Meguiar's Mirror Glaze #7 you do not need a dressing unless you like the permanent "wet" look.

Hope this helps...Lee