Value equality (a co-constraint use case)
Make the required/optional (or nillable/non-nillable?) property of a child element (or attribute?) depend on the values of one or more other elements or attributes.
source: Frank Langel 
Other use cases: Co-constraint Use Cases
a validation rule such as "if field a==21 and b==32 and field x==field z then c must be not null"
(Add your analysis here; see your name in pixels!)