Calculated fields let you perform one or more operations on an existing, pre-defined data field before it's printed or used within a report. They are especially useful in Summary/crosstab reports as well as in other custom reports where you want to do page breaks and/or sorting on something other than the built-in fields. There are four different types of calculated field:
Define calculated fields by using the Calculated fields link within the report.
Edit a field by clicking on it, right-click on a field to edit or delete it, and click on the links in the section to define an all-new field.