sum(field) sums all rows
sum(field,groupfield) will give you subtotal at each group instance. It must be placed inside the group sections (gh or gf) to show the value per group.
If you create a summary using the insert summary function and then drag and drop it from a Report footer to the group footer or header it will automatically add the group condition to it. That is not the case with a formula field that you create manually.
Edited by DBlank - 13 Mar 2012 at 5:11am