A Part Property is a property that specifies a part with strong ownership and coincidental lifetime of its containing Block. It describes a local usage or a role of the typing Block in the context of the containing Block. Every Part Property has composite AggregationKind and is typed by a Block. Part Properties are displayed in the parts compartment.
After setting a Block as a type for a Part Property the Composite Association is automatically created. You can see it in the Compartment area under the Relations node.