This rule will indicate a violation if any dark area is found on the layer containing the rule. Unlike most if not all other rules, no Region specification is allowed.
The Exist rule is intended for derived layers whose construction would indicate an incorrect combination of other layers (normal and derived). Layer expressions and derived layers can be used as alternatives to many of the built in rules, and for formulating new rules. The results are a bit different from the per-object and per-edge iteration of the normal rule evaluation flow. All violations are found as objects on the derived layer, there is no search limit (e.g., the normal flow may limit reporting to one violation per object, though an object may be associated with multiple violations). The approach gives the rule-author flexibility.