It is clear from Figure B.18 - Defining Structure of Power Subsystem (Block Definition Diagram) and Figure B.19 - Internal Structure of the Power Subsystem (Internal Block Diagram) that ChassisSubsystem.FrontWheel is shared by PowerSubsystem, and that FrontWheel is a specialisation of WheelHubAssembly.. However nowhere is it made clear which block is the composing owner of FrontWheel (although we may guess that it is ChassisSubsystem).

The polymorphic substitution of FrontWheel for WheelHubAssembly is never explained.