为什么在将png文件导入Flash CS3时会创建一个额外的符号？

When I import a .png file into the library in Flash CS3, I noticed that the system always creates an extra symbol in the library. If I import, for example, foo.png, into the library, it not only creates a bitmap with the same name, but also a Symbol of the type "Graphic" with the default name, e.g. Symbol 576. When I import a .png file into the library in F