When you import an Illustrator or EPS file, After Effects rasterizes it, converting it from a vector-based image to a bitmapped image. Depending on how you plan to use the image, you can choose to rasterize the image once or rasterize it continuously.
If you plan to use the image at its original size or smaller, you only need to rasterize it once, After Effects' default setting (Figure 5.39).