In general, each column of a report will contain values for a selected data field. However, here you can specify calculations that will derive the content of a column. You can only undertake calculations on columns that show on the report and cannot use a calculated field as part of a calculation.
Any existing entries are listed and you can click 
to create a further entry at the end of the list or 
to remove an entry that is not required. For each entry, which will define a particular column, input or select details at each field:
•Title provides a title for the data that results from the calculation
•Expression can either be input or you can click 
to use the Expression Builder, which is described next.
•Data Type/Format determines how the result of the calculation is to be presented. If you choose a Decimal type, you must specify the required number of decimal places.