This is the code of the Field that contains the data that will be displayed in the inquiry. Your selection is conditioned by the specific table that you chose in the previous field.
If you leave the field blank, you will have to enter an expression in the next column.
Note that, if the field is indexed (i.e., has several occurrences, such as the CCE field within the BPCUSTOMER table), you will have to reference a specific index, such as CCE(2). The indexes start at 1, not 0, so if you have 4 Dimensions defined in your system, you can enter CCE(1) to CCE(4), but not CCE(0) nor CCE(5).