The image shows the business rules editor in a Layout. The only business rule is the default form rule, which is selected. The Fields pane lists six fields. The fields are defined in the default layouts of two form. The "Show only fields used by layouts" filter is enabled, so object fields not included in the default layout are hidden.
In this image, two of the fields are highlighted. The highlighted fields were added to the list when a new rule set was created in the Layout.