Specifies if the element contains a reference Specifies that the reference element denotes a “null” reference. To be used only on elements for which sml:ref="true". A qualified name of an element in the referenced document. If true, requires the target element of the reference to exist in the model. A qualified name of the type of the element in the referenced document. If this attribute is set to true for a type D then instances of D should not create any cycles in a model. More precisely, the directed graph whose edges represent instances of D, and whose nodes represent documents that contain the source or target elements for instances of D, must be acyclic. References in URI scheme must be representend by this element.