GrandPerspective comes with a few default rules for masks and filters. When you edit a mask or filter, you can add your own rules using the New button from the "Edit Mask" or "Edit Filter" window. You can modify any rule, using the Edit button. Rules that you create or modify are maintained in the rule repository, which is automatically stored in your preferences. That way they are available next time you run GrandPerspective.
You can remove rules that you created, using the Remove button. You can also use this button to undo changes you have made to the default application-provided rules. The rule will then be replaced by the original rule with this name.