Grouping management in SSRS.
Group names can be used in expressions to specify scope for aggregated values.
=Sum(Fields!FieldName.Value, "Group Name")
=Sum(Fields!SalesAmount.Value, "Product_Category")
=Sum(Fields!SalesAmount.Value) & " of " & Sum(Fields!SalesAmount.Value, "Product_Category")
By default, SSRS will create a detail group which is a group without grouping column.
How to show the group pane: