Below are a couple of videos that explain how symbols can be added to other symbols to create motion and improve your animation. This process is also known as nesting. This is done to provide greater detail to animations. At times, it is not possible to get everything moving on just out stage in the way we are hoping to. Nesting symbols provides a lot more options and greater control.
Example Video One.
Example Video Two.
This is another video that you could also watch if you are interesting in creating characters that walk by nesting symbols inside of each other.
