The Field List window serves the following purposes:
1. Displays the list of all available data fields (attributes you can show in your report) and allows you to create report elements that will show information from these fields.
2. Allows you to create calculated fields by building expressions based on the values of data fields, report parameter values, etc.
3. Shows existing report parameters and allows you to add parameters to your report.
If the Field List is hidden, you can enable it in the Main Menu, by selecting View/Windows/Field List.