The screen capture above shows the "Sort Expression" dialogue box (also known as the Break Properties" dialogue box). This screen is accessed by double left-clicking on a sorting band; it's purpose is define what database field will be used to sort the report output.

 

To add/edit the database field that will be used in the sorting band click the blue button next to the entry field - this will display the Dictionary, which is the easiest method to select the desired database field.