The macro-alias element in macro-ref must be unique. The macro attribute of macro-satisfied must refer to a defined macro alias. The composition-alias element in composition-ref must be unique. The composition-alias-ref element of a select element must refer to a defined composition alias. If the select element contains an extension reference, the SIS implementation will fire an event to the extension component, using the SCS API to select a composition. If the select element contains a composition-alias-ref, the SIS implementation will invoke that composition. If the select element contains a profile-lookup, the SIS will invoke the profile lookup method to find a composition. This element specifies an implementation-specific method for looking up a composition using profiles. The scheme attribute specifies which type of profile lookup will be performed. Available schemes must be documented by the SIS implementation. The key attribute specifies the name of a field from the initial request. The value of this field will be passed to the profile lookup implementation. If this terminate element is present in the instance document, the SIS will perform the implementation-specific composition termination action. Protocol-specific schemas may replace this element with something more useful, by defining an element that extends TerminationType and is in the "terminate" substitution group.