Imagine you have an Purchase Order workflow with a Table control. The Table has Price,Quantity and a Subtotal column. The fields are named Item_Price, Item_Quantity and Item_Subtotal. You want to multiply the Item_Price times the Item_Quantity and display the results in the Item_Subtotal fields for all the rows in the table. The Rule Builder automatically creates the code to handle adding/deleting table rows.
The sum() function mentioned above will calculate over the entire repeat/table. To use addition to subtotal individual rows, use the "+" operator, i.e. Set Subtotal to Item1 + Item2.