The GUID for the asset that the referenced object belongs to.
The assetGUID constitutes the {a} element in the string representation of a GlobalObjectId, the format of which is:GlobalObjectId_V1-{i}-{a}-{l}-{p}
The GUID is a globally unique identifier that Unity assigns to every newly discovered asset. For more information, refer to Asset Metadata in the Manual.
Additional resources: AssetDatabase.AssetPathToGUID, AssetDatabase.GUIDToAssetPath, AssetDatabase.TryGetGUIDAndLocalFileIdentifier