Thank you for your post. Each purchase order acknowledgement (855) is specific to the retailer so the field for the back ordered information may be different for each retailer. Which trading partner are you working with? I can take a look at their form and provide you with the specific place.
If you have any other questions please let me know!
The retailer is Wayfair. But don't worry - the awesome Mr. Engrav at SPS Commerce answered my question. For anyone else having the same issue, here's his response:
When you have the 855/PO Acknowledgment open, if
you go to the Detail tab, on the right there's an "Acknowledgment"
column. If you click on the first box in that column, in the new window that
opens up, you can select a Line Item Status of "Item Accepted -
Backordered", and then in the Current Schedule Ship Date column, select
the date there.