アセットが外部アセットかどうかを判断します。
A foreign Asset is an external file that you import into your Unity Project (for example, a .png Texture file is a foreign Asset).
Upon importing a foreign Asset, Unity will produce a serialized representation of the data in the Project's Library folder. This is done when the Asset is first added to the Project and when the foreign Asset is updated externally to Unity.
See Also: AssetDatabase.IsNativeAsset