You can use a Class with Ports as a part Property in a higher level system.
If the context using the Class only every interacts with the "instance" of the Class via its Ports (with the provided/required Interfaces as declarations), then the Class is replaceable within the context, promoting true design-by-contract and service orientation. The part Property can thus play a replaceable "role" within the higher context.
TIP: In MagicDraw UML one can also use a Class with Ports as the type of a Port to create nested ports, a powerful systems engineering feature !
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