3-Plate Method, Surface Plates?

1. What is a surface plate?2. What is the difference between cast iron and granite surfaceplates?3. What are the advantages of granite over cast iron surfaceplates?4. How are surface plates graded? How flat are grade Asurface plates compared to grade B?5. What is the three plate method, be specific?6. How is a surface plate calibrated?7. Why are inserts (T-rail(s), Threaded plug(s)) installedbelow the surface of the surface plate? (Note: they areusually about .050 to .100 inches below the surfac