The Optional Fields tab in the Setup Report window shows the additional output fields you can include as columns in the current report.
Some categories will be collapsed underneath headings in bold. You can use the + and - icons to expand and collapse these sections. The blue icons here can be used to tick and untick all options within a section.
Report Output Hierarchies
Report Templates that are based on other reports will have the optional output fields for the master report also available to add. For each report in the Report Template field, the optional outputs from the Master Report will also be available to select in the window shown above.