When you collapse an object of the Call activity type a model of the BPMN collaboration diagram (BPMN 2.0) type is created.
If you want to create an assignment to a model of the BPMN process diagram (BPMN 2.0) type, you must create an assignment manually. Only models of the BPMN collaboration diagram (BPMN 2.0) type are offered in the dialog for selecting a global reference.
If only one assigned model exists, the objects of the assigned model are displayed in the object of the Call activity type when an object of the Call activity type is expanded.
If an assigned model of the BPMN collaboration diagram (BPMN 2.0) type exists, as well as an assigned model of the BPMN process diagram (BPMN 2.0) type, the objects of the assigned model of the BPMN collaboration diagram (BPMN 2.0) type are displayed when an object of the Call activity type is expanded.