Dear,
Back order processing is the processing of a backorder, which in itself is a sales order that has not been confirmed in full or not confirmed at a certain delivery date.
Tcode: V_RA
Rescheduling Tcode: V_V2
Backorder Processing is a functionality in SAP where you can change the commitment and over-ride the blockage of stocks marked against sales documents/deliveries. For example, you receive an order from a very important customer for material "A" but the entire quantity of A is committed to another customer "B" via earlier sales orders and this is where Backorder processing helps you to change the committment and shift stock due for B to A. This is the benefit of this functionality.
Please refer the below link too,
http://www.gotothings.com/sapsd016.htm
Regards,
R.Brahmankar