Main asset that was supplied when building the asset bundle (Read Only).
This is a convenience function that makes it easy to find the primary asset of a bundle. For example you might want to have a prefab of a character and include all textures, materials, meshes and animations files with it. But the fully rigged prefab of the character would be your mainAsset and can easily be accessed.
function Start () {
var www = WWW ("http://myserver/myBundle.unity3d");
yield www;
// Get the designated main asset and instantiate it.
Instantiate(www.assetBundle.mainAsset);
}