The Rules That Apply Here Pane
The Rules That Apply Here Pane is a list of every css rule and associated value that applies to (will affect the display of) the currently selected element or element type (this element or type is selected with the Choose Parent and Choose Target Child menus).
Next to each rule value is a link to the element where the rule is set. Clicking this link will take you to the Edit CSS Rules Pane for the element that defines this rule.
As your form gets more complex, this pane will allow you to pinpoint exactly which rules from which parent element(s) are affecting the display of the selected element.
In the graphic above, you can see that one of the css rules for this element (a Text Box with a Label attribute of "TextBox One") is set locally by the parent container (border-color which overrides the border-color set for Child Container element types by The Form), one is set by the parent fieldset (padding which overrides the padding value set by The Form), and the rest are set by The Form.