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You can select the Backordered Qty of the items and click on Add Request button to create a Backorder for that order. (note that there may be some ordered qty has been put into Backorder, and they cannot be allocated until you send them back to fulfill).
Qty to Backorder = Assigned Qty - Prepare Ship Qty - Backordered Qty.
Prepare Ship Qty: Qty of items have been put into the fulfillment process.
Backordered Qty: Qty of items have been created backorder request.
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After the backorder request is created successfully, a toast message will show “You have created a backorder request successfully!”.
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