Scenario
You have promised several sales orders based on the quantity and inbound delivery date of a purchase order. Now the vendor of this purchase order informs you about a production delay. In addition, only a partial delivery will be possible.
You want to reflect these changes in the confirmed delivery dates and quantities of your sales and store orders.
Backorder Processing - reATP (Rescheduling)

Relevant customizing is maintained in IMG → Cross-Application Components → Advanced Available-to-Promise (aATP).

The use case for reATP is to re-confirm orders (sales orders and stock transfer orders) if supply and demand has changed (significantly). Previous ATP confirmed dates and quantities get replaced if necessary.
In the example above two orders had been confirmed against future supply (a purchase order). Later the purchase order had been updated because the vendor faced some production capacity issues resulting in a later delivery with a lower quantity. ReATP therefore confirms both orders for the new stock availability date. Being of lower priority the sales order can not be completely confirmed completely any more.