When creating a custom report you can append additional filter criteria to the SQL WHERE clause to create ad hoc filters. You can also update the WHERE clause itself, which reflects the filters selected in the Filters page of the Report Wizard. The SQL statement shows you the tables you can use for your filters.
To edit the SQL for selection criteria
Additional Information
Select Custom Report Grouping Information
Select Custom Report Column Order
Select Custom Report Column Filters
Select Optional Tables for Custom Reports (Advanced)