The column name or expression expression.
...
<Columns>
<DNL:GridColumn ColumnExpression="orderid" Label="Order ID"/>
<DNL:GridColumn ColumnExpression="(quantity * price) as value" Label="Order ID"/>
</Columns>
......
<Columns>
<DNL:GridColumn ColumnExpression="orderid" Label="Order ID"/>
<DNL:GridColumn ColumnExpression="(quantity * price) as value" Label="Order ID"/>
</Columns>
...