This question was raised in the 'How to implement a secure IoT system on ARMv8-M' webinar, view all the questions in the round up blog post.
Thanks Mike. Nice to hear from you! (I wasn't able to explain the difference as clearly as you guys could. Thanks for the assist! But there's one answer I couldn't find -- is there any restriction that is imposed on the IDAU by the existence or construction of the SAU?)