DO_SUM is very helpful when you want to assure that some of the numeric fields are summarized directly after the output, without interaction from the user side. In order to do so, just put the ‘X’ or ABAP_TRUE ...
As you can imagine from the name, NO_SUM does mostly opposite what DO_SUM, but in more secure way. If you don’t want that the field is allowed to be summarized, then set the value of this field to ‘X’. ...