Mapping EDI PO acknowledgement (855) and ASN (856) from vendors

Does anyone have a recommendation on which record to use for mapping inbound EDI PO acknowledgement (855) and ASN (856) from a purchase order (850) to a vendor?

I assume for the 855, if the vendor is confirming & accepting quantity / price / ETA for a purchase order we sent to them as an 850, that could logically be mapped to Purchase Order custom body and column fields.

But for the 856, the tracking number and items & qty's inbound, when there's a chance of a one-to-many relationship between PO (and PO line items) and item receipt, it may not make sense to map directly to the Purchase Order record. Would it make more sense to have an Item Receipt in an "in transit" location, and then edit it at the actual time of receiving? Or custom records for inbound shipment lines?