Define Composite Elements
A stereotyped element can be created automatically as a composite element. You can define this, and whether the child diagrams of the composite are of a specific type, using special attributes.
To define whether an element is always made composite on creation, you apply the _makeComposite special attribute to the appropriate metaclass element (not to a stereotype element). A stereotyped class, when created, does not default to having a child diagram, so you use the _makeComposite attribute to trigger creation of the child diagram. For a stereotyped composite, the child diagram is of the usual default diagram type for the metaclass; you can change the child diagram type using the _defaultDiagramType special attribute to identify the preferred diagram type,
This example from BPMN shows that a BusinessProcess element is always created as a Composite element with a BPMN custom child diagram.