I think layering and separation basically comes down to the idea of avoiding clutter. You have to be able to combine a variety of elements into one finished product. However, the trick is that the audience has to be able to comprehend it and it must reveal the point you want to get across. Tufte gives an example through the 1+1=3 principle. Once two black lines are drawn, a third line or white path is created. Normally this type of thing is not wanted. This is a simple example, but it applies to information design as a whole. When you place two objects together sometimes it will not have the desired effect that you thought it might have. I think this quote sums up layering and separation, "Failure to differentiate among layers of reading leads to cluttered and incoherent displays filled with disinformation, generated by the unrelenting interactive visual arithmetic of flatland, 1+1=3 or more" (Tufte 65).