Solution : https://service.sap.com/sap/support/notes/549146 (SAP Service marketplace login required)
Key words : 
define customer-specific subtotals, customer-specific formulas, customer-specific subtotal, define subtotals, user exits, terms setobjectforuserexits, formula reason, methods setobjectforuserexits, pricing item, goup conditions
Related Notes : 
       
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