The Modify Constraints control shown below is accessed by pressing the Add/Remove Constraints button within a spec section. This control enables you to specify the fields that will be available in that section to define constraints.
Add a field to the Selected section by selecting it from the Available section and pressing the ' > ' button. Likewise, remove a field from the Selected Fields section by selecting it and pressing ' < '. The changes will be applied click on an area outside of the control.