DIVE DEEP INTO SUMMARY FIELDS with Dwayne Wright:
One thing which is often confusing to new users is a summary field calculates on the current found set or the grouping that applies to a sub-summary part. This means you have to take into consideration the "found set" when you have summary fields in reports that run from scripts. Say, for instance, you have a summary field for sales named "Sum of Sales." It's the sum of a field called Subtotal. We used the Subtotal field because we don't want tax or shipping amounts to show in our sales number. If you perform a find for sales in 1998, this field will show the sum total for those records. If you search for June of 1998, it will only result in the total for those records.
Most beginners don't realize you can have a single sub-summary field (subtotal, for example) show different results on the same report when multiple sorts are applied to the found set. Once you figure that out, you get much better reports.