This is a bit of an oversimplification, but when a part gets anodized, the anodize layer forms halfway into and halfway above the original surface. So if a drawing calls for 0.002″ of anodizing, then the part will “grow” in size by 0.001″ per side of the part. For example, if a raw part is exactly 1.000″ thick and requires 0.002″ of anodizing, the part will be 1.002″ after plating, not 1.004″ after plating.
Standard anodizing (Type II), is typically 0.0004″ thick, adding 0.0002″ of thickness per side to the part. Hard coat anodizing (Type III) is typically 0.002″ thick, adding 0.001″ per side. For both Type II and Type III, the plating thickness can be controlled by shorter or longer anodizing times.