What is the expected functionality of inbound purchase order EDI code 856 for ASN?

Summary:

Our business exchanges purchase order communications via EDI but we have never enabled nor tested inbound 856 from our suppliers.

We understand that this is a supplier's way of communicating that a purchase order shipment is on it's way, as well as details on items and quantities within that shipment, but is that all it does?

Is it possible for the ASN communication to automatically update a purchase order lines requested delivery date or promised date?

Also, if an ASN has been transmitted, does the receiver then have to receive directly against the ASN number? Or can they receive against the purchase order number (as they do today)?