Why need SumIndexField?

SumIndexField can be defined against the table keys. why we need to define this? OK! let me explain. As a example we can define secondary key like (Document Type, Document No., Type, No.) in Sales Line table. we can define SumIndexField against to this secondary key like(Quantity, “Total Price (LCY)”). After define SumIndexField SQL Server creates a View for it automatically. Following SQL Query represent the query of the above view.