The ID of the asset.
This should be unique amongst all assets with the same extractable_type. Depending on the associated Extractable implementation, this id may convey some information about the Object that should be extracted. Note that, as such, the ID will have an expected format, and you can not choose this value arbitrarily. By default, this will be set to the type name of the extractable_type, but you should check the documentation of the extractable type to see whether they differ from the default behaviour.