This attribute is only usable for the root element, with x:Class specified for that generic type, and only when that root element maps to a generic type with at least one type argument. An example is PageFunction<(Of <(T>)>).
If the generic has a single type argument, the attribute value is a single type reference, with type specified as attribute value using the x:Type Markup Extension syntax. If the generic has multiple type arguments, the multiple type references are separated by commas within the attribute value.